You're reading this guide because you want or need to buy a tool - even if you have lots and lots of them - for one of the following reasons: 1) You broke one, 2) You need it, or 3) Just Because (my personal favorite). My experience in wrenching with Snap-On, Mac, Matco, Williams, Fleet, Ward's, Challenger, Blackhawk, Bonney, KD, Proto, Allen, Husky, New Britain, Craftsman, Stanley, no-name Chinese tools, no-name Taiwanese tools, no-name Japanese tools, etc., may be of some use to you.
Your first problem is figuring out how much to spend. Your main crunch (if you've got one) would probably be more than happy if you didn't spend anything at all - there's too much crap in the garage already. OK, let's just say it's always nice to spend less. So how much less?
Probably the most important question is how often are you going to use this tool. The guys whose livelihood depends on their tools will generally use Snap-On, Mac, or Matco; Craftsman, Proto and S-K also show up on a regular basis. There is a reason for this - cheap tools have an annoying tendency to wear out and/or break (we'll talk about the breakage factor later). For example, if you're going to use a 24mm socket on a daily basis, it makes more sense buying the best you can afford - the higher cost will even out because they will (usually) last longer.
The second question is just how picky you are. Those that buy cheap tools have never known the feeling of holding a great tool in your hand - it will be balanced, heavy, and Just Feel Right. Holding (and using) a really good tool is roughly equivalent to caressing the metal equivalent of Claudia Schiffer. If you can get hold of an 18" Snap-On 1/2" drive breaker bar, you'll see what I mean. My obsession with Snap-On began when I bought a 10mm combo wrench for $5 - it had seen massive use, with lots of chrome worn off, but it still grabs like mad.
The third question is simply - are you going to use this tool to do something stupid? As an example, back when I was young and stupid (instead of just young), one of my first wrenching experiences was changing out a drive axle in my first-generation Honda Civic. Axle nuts are notoriously difficult to budge, and this one was no exception. I finally put a three-foot-long piece of pipe over the ratchet handle, and slipped a six-foot-long length of fence post over that. And jumped on it. Had the socket and/or ratchet been anything less than the Williams items I was using, something would have broken (probably my head). And if you break something, it's nice to have a warranty. Craftsman, for example, can be easily changed out at any Sears store. Don't be embarrassed, they're used to it, and the clerks won't interrogate you to see if you've been using that screwdriver as a pry bar. From what I understand, Husky has a similar return policy at Home Depot. Snap-On tools are easily exchanged (although you rarely hear of one breaking) and those trucks show up almost everywhere. Many other brands have excellent - even lifetime - warranties, but just try getting it honored - you'll probably have to find the manufacturer and send it back, and I find that just more trouble than it's worth.
The fourth question is - is this tool safe from theft? Tools are even more prone than car keys to wander off when you're not looking, and the more expensive the tool, the more likely it is to wander. You may not go as far as said in one Snap-On sticker ("Want to borrow a tool? The last guy who borrowed one is in the bottom drawer"), but you'll probably want to have some sort of permanent identification on your tools, and if you expect it to walk off soon, try not to spend too much money on it.
So what are you going to buy? The first rule is - avoid cheap. Period. There is a reason you can buy a "complete socket wrench set" for $5 - it's built cheap from cheap materials. Those sets will make you cry - when it breaks, you haven't finished the job and the store's closed, OR when you put on some torque, it breaks and your knuckles slam into something. Your first clue a tool is cheap is that it feels too light - experiment with holding, for example, a nice S-K 1/2" drive socket in one hand and one of those no-name sockets with thin chrome plate and a perfectly round little hole in the center in the other. Feel how much heavier the S-K socket is? Steel is expensive. Many cheap tools are made of inexpensive alloys, apparently comprised equally of aluminum, wet newspapers and old tuna cans.
The second rule - looks aren't everything. There are a LOT of tools out that that try to look expensive, even mimicking the look of some high-end brand (swap meet sellers of Snap-On lookalikes are notorious for this). When in doubt, look for deep engraving or forged markings - the barely visible stamped words or numbers (or complete lack thereof) on cheap tools can be a dead giveaway. On the other hand, many old or well-used tools will have their markings worn off or originally possessed no markings whatsoever. Use your judgment.
The third rule - never buy something that can hurt you. If you live far from a tool vendor, a broken tool can render your whole weekend useless. Or, more likely, you'll be putting serious torque on some tool, and it will either slip or break. In either case, the end result to your hand (or head!) will look and sound roughly equivalent to dropping a ripe cantaloupe off a 10-foot stepladder. Your knuckles are expensive. So are teeth. I was under my Olds 98 (yes, with that tufted velour cushion interior) many years ago, pulling down to remove a recalcitrant starter bolt, when the cheap no-name India-made wrench I was using rounded off the corners of the bolt and smacked into my face. One split lip and a chipped front tooth later, it occurred to me that I had learned something.
The fourth rule - forged over cast. Cast anything breaks more easily, so cast tools will usually be fatter, clumsier and not as strong as their forged equivalents. An example was when I was working on my late, lamented 450SL - my 40th birthday present to me. Being generally cheap and/or broke, for years I refused to buy a new hydraulic fan clutch - the dealer price of almost $300 probably had something to do with it. And you can buy a used fan clutch at your local Pic-n-Pull for about $10, which will usually last about a month. Unfortunately, the 10mm bolts holding the fan clutch on were cleverly recessed. Consequently, once the edges of the bolts were rounded enough that my cheap open-end wrench wouldn't grip, I tried using the box end. No dice. The cheap cast wrench I was using had a box end too thick to fit in the recess. I was faced with the prospect of removing the radiator AND the condensor AND all the associated hoo-ha in the way in order to get the fan clutch off. Earlier in my wrenching career I would have given up, and taken the car to the mechanic. That time, I got smart - I went to the local hardware store and bought a nice "Master Mechanic" forged and polished combo10mm/11mm box-end wrench for about eight bucks. And guess what? The forged box end, much thinner than the one on the cheap cast wrench, slipped neatly into the recess, grabbed the rounded bolt head slick as you please, and the job was done in a few minutes.
The fifth rule - balance your time versus your money. If you can wait to get the tool, AND if you have the time to spend on hunting for it or them, hold out for good used tools vs. new cheap ones. Ebay, and sometimes Craigslist, is a great resource for this. A compromise, should you need the tool immediately, is buy Craftsman, Husky, or KD - they are universally available, generally well priced and (almost) always priceworthy. If you need a tool to work first time, every time, spend as much as you can afford, you'll thank me later.
The sixth rule - they're cheaper by the dozen. Almost always, you can buy a full set of whatever tools you want for less than the individual prices of each item included. Once again, Snap-On is the exception to the rule here - there's no discount to buying them in bulk. But if you buy a Craftsman set (to use an example), you'll pay as little as 50% - sometimes less - of the aggregate cost of buying their individual components. And make a habit of watching for those Sears sale brochures that show up in your mail (I'm dating myself here. Usually you can just sign up online for "Craftsman Club") - buy 'em when they're on sale.
The seventh rule - it's fun to hunt on Ebay. Keep track of what is selling, use the "advanced search" to find what it typically sells for, and you'll be able to better predict what a good bid will be (and if you can afford it). I've bought more than one tool on impulse (particularly in the "Collectibles" section), and several have turned out as favorites. I've built up my Snap-On collection by bottom feeding over a period of some 10 years, usually bidding no more than 1/3rd of retail.
The eighth rule - BUY IT BEFORE YOU NEED IT. If you aren't rushed, you can wait for those "once in a lifetime" deals to come along - I have a gorgeous set of Bonney 1/4" drive sockets that I bought for $2; they've come in very handy. Ditto my 2nd set of offset closed end Snap-On wrenches - I bought them for about 20% of retail, and the following week I had a series of bolts to deal with that had insufficient clearance for a ratchet and hellish access that made the offset priceless.
Finally, some random thoughts about various common brands and items, in more-or-less descending order -
Snap-On - the holy grail of American automotive tools. In quality and price, overall the highest. You definitely get what you pay for here. For example, Snap-On uses a process on their screwdriver tips that make them both highly accurate and virtually indestructible - if you have a screw with a damaged head, your best chance is a Snap-on screwdriver. Break it, your Snap-On guy will replace the shaft. If you happen to come across one of their ratchets for cheap, buy it - you can have the innards replaced for next to nothing. I have a 1/2" Snap-On ratchet, date coded 1948, that I bought from a closed tractor repair shop out in New Jersey - the chrome, except for 1 or 2 specks, is gone - it works great. Imagine the amount of work that tool has done in its lifetime. Also, Snap-On screwdrivers generally have a hex section of the shaft that you can slap a wrench on for greater torque - very handy.
Mac, Matco, Stahlwille - pretty much the same as Snap-On. For whatever reason, I've never fallen in love with a Mac or Matco screwdriver. My creeper is an old Mac, the wheels have held up well and it's a solid piece of work.
Williams - also known as J.H. Williams - heavy, solid and virtually indestructible, the -50 and -51 ratchets built America's bridge, pipeline, and general infrastructure. Williams was THE industrial-use tool maker of choice for over 75 years. Williams is now, btw, a division of Snap-On, though I haven't seen any difference in the quality of the tools. Williams "Superratchets" in the -52 (and higher) series have remarkably fine ratchet mechanisms - important if you're in a tight spot with very little wiggle room. My favorite ratchets are all Williams -52 series. Earlier ones, -51 or -50, while having coarse mechanisms, are great for heavy duty work. A set of Williams SAE 1/2" drive sockets, with their accompanying S-52 ratchet, were given to me about 1980 by a factory maintenance tech who had used them day in and day out for more than 25 years. I've owned them now for more than 30 years, and they've never let me down. I also have a SAE set of Williams offset closed-end wrenches made in the '30s, which came from a closed auto repair place off Route 66; very nice stuff with quality, patina and history. If you can find one, there is an odd-looking all-metal Williams 1/4" driver handle that will 'click' into two positions - one will allow the shaft to spin (to use as an extension), the other will lock the shaft in place to use as a driver. Also, if you're going to put serious torque on anything, Williams extensions with the knurled rotating handles are priceless.
Proto, S-K, Napa, New Britain, Allen, Klein, KD, GearWrench, Channelock - Always reliable, never cheap. Allen, KD, GearWrench, and Armstrong are now all part of Apex Tool Group. Although very well made, S-K ratchets generally have too coarse a mechanism for my taste, try before you buy. Klein makes damn nice screwdrivers, btw; some great gripping tools too. For work where you don't have a medium or deep socket that fits over the bolt in question, a GearWrench (or equivalent) makes life much easier.
Plomb aka Plumb, Bonney - Plomb was a U.S. brand (that was made right here in Los Angeles around the time of WW2) renowned for their "pebble finish" wrenches. Very nice ratchets with (usually) a very soft "drag"; everything of theirs I've ever owned has been built like the proverbial tank. If you can find old Bonney wrenches (mostly SAE, I've seen very few metric) they are solid as all hell - Bonney had a guarantee that a Bonney wrench would break the bolt before the wrench gave out.
Craftsman - I used to think that Craftsman was, bar none, the best value for money. Lately, I'm not so sure. A few years back, Craftsman changed manufacturers, and the newer stuff (compare your old Craftsman sockets to new ones, example) just doesn't seem to be of the same quality or to hold up as well, particularly their Phillips-head screwdrivers and cutting tools. The combination sets tend to include their really cheap ratchets, which are coarse, clunky, with a piss-poor handle shape (like bar iron, with the narrow part digging into your palm) and just generally a pain in the ass. Craftsman's polished "Professional" line of wrenches is nice. I have yet to meet a tool in their lower-cost "Companion" line that I liked at all.
Crescent, Husky, Stanley, Great Neck, AutoZone and Pep Boys house brands - usually reliable, not particularly expensive, in my experience usually - but not always - a notch or two below the run-of-the-mill Craftsman stuff. I've never had one of those brand tools become a favorite, with the exception of my lost, much lamented Husky impact driver that is now reposing somewhere deep in the wall behind my built-in tool drawers.
Generic "made in Taiwan" tools - in my experience, usually shoddy, sometimes good, sometimes excellent. Judge these on a case-by-case basis, sometimes an excellent value for money. The better ones will usually be stamped "chrome vanadium" or something along those lines. I have a generic Taiwanese stubby ratchet marked "chrome vanadium" that is beautifully built; it has gotten me out of more scrapes than I can remember.
Generic "made in India" tools - usually shoddy, occasionally o.k.
Generic (mainland) "made in China" tools - some decent, but mostly crap or worse. A few, including those sold in Pep Boys and Auto Zone, aren't bad for the money, especially if you're not going to be doing anything particularly demanding with them.
Generic tools trying to look expensive - don't bother. Ever.