Title says it all. What's the best, most useful tool you have used?
For me it's a 39 piece screwdriver-adapter set that my parents gave me as a present when I was just a teenager. It has hexes, sockets, screwdrivers, and more, and the little organizer keeps everything neat and convenient and gives you a place to put removed screws and such. It's also decent quality, not some flimsy gonna-break-in-a-year type stuff. It's the first thing to come out of the box for most projects.
So how about you? Is it your leatherman? Or maybe just an old fashioned pocket knife? That old-school forged-steel pair of pliers that's been passed down for three generations? Some high-tech piece of gadgetry? Or maybe your chainsaw?
I've never found much use out of Leatherman-style plier multitools. The screwdrivers in them are practically useless. The pliers and cutters usually work OK, but are less comfortable to use than a normal pair of pliers/cutters. They also break very easily-- I've broken two and I barely use them.
Similarly, I find that Swiss Army knives are one tool that I rarely use and never enjoy using. When I need a knife, I need a real knife, not some flimsy garbage on the side of a cheese box. It does a dozen things poorly.