Hands down, best tool I have owned so far is the Topeak Ratchet Rocket. This tool has almost everything I need, and not a bunch of extra crap like some of the multi-tools I've owned, dull knifes, bottle openers (ahem, alien?). In addition, I carry around a small adjustable wrench, spoke wrench, a CO2 inflator, tire levers and a patch kit (glue tube type). I should be carrying around a spare tube, but I've been feeling lucky ever since I put Stan's No-Tubes in my tires (presta and schrader types), which is clearly hubris and now I'm just asking for 3-4 flats on my next ride.

A word about the Rocket Ratchet (I love this thing): it solves the one problem that has seemed to elude every other multi-tool. When you crank on a nut with a standard multi-tool (the ones where all the tools are threaded together and sandwiched between two plates by a screw), that force inevitably and imperceptibly loosens the screw holding the whole thing together. So you use your tool, and put it away till the next time you need to adjust something, and the vibrations from the bike and the torquing (can't believe that word made it past spell checker) you did earlier, the screw came loose and all the individual hex wrenches are floating uselessly in you bag until you can find ANOTHER hex or screwdriver or whatever to repair your tool so you can repair your bike. I don't know about anyone else, but it's happened with every multi-tool I owned. The solution for me was either: a) buy a set of mini hex-wrenches, which was kind of a pain and not as cheap as you'd think, especially if you need a T25 wrench or b) get the Rocket Ratchet, which is self contained, cheap as any other multi-tool and has a ratchet! It's the only multi-tool that seems designed to do what it was intended to do. It's got a couple neat features, like a socket on the non-ratchet end of the wrench part, so you get as much/more reach than a standard multi-tool. Downsides: you have to switch out bits like a normal ratchet, so they can get lost if you have butter-fingers, and no flat head bit, but I haven't needed one, but it could easily be bought and carried along side.

Oh, and a pair of latex gloves. Those are nicer to have than you might think...