Adjustable wrenches have their uses, but applying the most torque you can is not one of them. If you're limited to buying one type of wrench and you're dealing with small fasteners, you're better off with a set of combination wrenches. I'd look at the now-common ratcheting ones if I were starting over, but I haven't had a chance to try them.

For larger hex things, and for square things, adjustable wrenches work fine. They're much nicer on (hex) pipe fittings than pipe wrenches.

I find the "adjustable" moniker doesn't really create the best expectation of how to use them though, implying that you choose the right setting to do the job. On every use, I back the jaws off slightly with the thumb screw, put on the piece, then tighten up while it's on the piece. The more torque I need to apply, the more deliberate I am about making sure the jaws are snug.

Vise grips are for when you would otherwise use pliers, and want them to latch.