If you have the ability to carry an edge, multitools offer a ton of utility in a small package. Some people prefer carrying a small knife instead of a multitool, but that much specialization, especially on a keychain where space and weight are a strong premium, seems silly to me, the EDC version of a LOOGY in the bullpen. Granted none of the multitools out there have blades that match the ease and performance of, say, a Spyderco Ladybug. When comparing keychain multitools, I strongly prefer the presence of pliers in a multitool, so I have to give the edge to a Leatherman-type tool. That said, a SAK Manager or an alox Cadet could work well on a keychain. Of course other companies make keychain multitools. Here are a few:
Leatherman (many makes and models)
Buck (the Mini Buck and the Metro)
SOG (Cross Grip and Cross Cut)
I have tried very few of the options out there, limiting myself to about three different Leatherman Micras, a pair of SAKs, and my beloved PS4. Generally I like the SAK for typical urban/suburban use cutting open blister packs, slicing an apple (the SAK steel is soft as butter, but virtually rust proof under normal use). I prefer the Leatherman when I know there will be some home improvement or repairs to do. In a funny, let's-see-if-this-will-work moment I replaced the entire ball chain switch assembly in a ceiling fan with nothing more than my PS4, even using it to cut and strip the wires. The PS4, in my mind, is the perfect micro multitool:
Instead of reviewing every multitool out there, I am going to look at what features you need to have and then move on the size/weight issues.
If you are looking for a device that will make your urban life easier then you can probably skip the pliers. I love pliers (their like opposable thumbs without the annoying fleshy coverings and nerve endings), but they are not strictly necessary for the urbanite. So pliers are either a must or an avoid and will likely determine which route you take--SAK or Leatherman-type tool.
No matter which route you go down it is silly not to avail yourself of the convenience of a folding blade. Even a tiny 1 1/2 inch blade will work in 90% of the cutting tasks you will encounter. If you fly a lot or go into other secure areas a blade is a no-no, but otherwise, why bother carrying a true multitool if you aren't going to get a blade in the process. I like the steel on SAKs for general everyday use. I like the exterior blade of Leatherman tools, as opposed to those that require you to open the tool's handles to access them. Serrations are never necessary, but if you can get a tool with two blades one fully serrated and one plain edge you'll be better off. Blades this short don't work well when they are partially serrated.
Next, I would look for a pair of drivers, again a Phillips and Flat head driver. It would be nice to see a hex bit, given its prevalence in the knife world, but it is hard to make those bits both flat and effective (unlike the "2D" Phillips bits that work pretty well). SAK drivers, especially those on the Tinker, are really great. The full sized Phillips driver is very nice, especially in the T handled configuration (allows for a bit more torque). The problem is a SAK the size of a Tinker is too big to put on a keychain feasibly. Instead look for something like the SAK Manager, seen here:
The last "required" tool, in my mind, is a good pair of scissors. The PS4 has a tiny, but functional pair. The SAK version of scissors is excellent, besting the PS4's, but also being significantly larger. Scissors are just too useful and the number of tools with both pliers and scissors is small. In fact, only the PS4 has both in a package sized for keychain carry.
Other nice features: one hand opening knife, a pen, a can opener, a rasp or file (again the PS4 has a decent file). The nail clipper, the reamer, the hook...all of these are stupid and should be skipped unless you have a specific application for them. Unnecessary size and weight. Of course if you don't have a bottle opener on your keychain in another tool, it is an absolute requirement.
Keychain carry means different things to different people. Some folks carry a Leatherman Skeletool on their keychain. That seems a little silly to me. I can't imagine carrying that around all day, lugging it in and out of my pockets, and having all that weight hang on the ignition.
Practically speaking something larger than the SAK Manager is not feasible. Even something like the alox Cadet, which is slim as a pin, is still too long. I also think that the Leatherman Juice and the SOGs listed above at still too big. If it needs a sheath to work it is too big. If it weighs the same as your keychain all on its own it is too big. In my experience, the Micra is just about the perfect size.
This is a personal preference, obviously, but think about this--you will ALWAYS be carrying this around. If you are out of your house and not in your yard or something like that, at least part of the energy you are expending AT ALL TIMES will involve carrying your keychain around. What if you had to carry around a two pound weight in your pocket so that you could exercise 10 minutes a day while at work? No one does that, even exercise fanatics. That sounds ridiculous, but 10 minutes a day is lot more use than you will get out of your keychain multitool most of the time. Think about that 2 pound weight the next time you think you need a bigger device on your keychain.
If I was starting over, I'd still pick the Leatherman PS4. The combination of pliers and scissors is really nice. The fact that all of the tools are exterior mounted is awesome. It is the right size and price. If I was going to go the SAK route, I might be tempted by the Manager, which is a favorite of a lot of folks, but the recently released Signature Lite seems like a winning package for those that aren't fix-it oriented. A pen, blade, light, and scissors with the same dimensions as the Manager.
It all comes down to what you are going to do. I like to work on my house and build stuff out of wood. I use pliers a lot. For me the choice is clear. Pliers=Leatherman; no pliers=SAK. From there your jobs dictate the choice, but a blade, drivers and scissors are must haves. There are a lot of choices out there, but only a few good ones.