I hate multi-purpose things with a passion. Combining a handful of crap short allen keys with some half-decent cone spanners, chucking in a spoke key, bottle opener and flat screwdriver, and trying to fit everything in a tool the size of a small cellphone just doesn't cut it in my books.
Allen keys: 2.5, 3, 4, 5, 6, 8
Spanners: 8, 14, 15, 17, small and large adjustable ones
Screwdrivers (flat, Philips and Pozidriv), Mole grips, mallet, BB tool, cassette tool, chain whip, spoke key (park), puncture kit with good tyre levers, gaffer tape, threadlock (loctite) superglue and a block of wood is alll I ever use for working on either of my bikes.
oh yeah, if you run maggies/hydraulic discs, a suitable bleed kit and a bit of fluid isn't bad to have.
buy expensive tools, it is worth it. Bring a bag on longer rides.. You might think you would like to go bagless because of the freedom and only carry a little multitool, but if you have a bag you don't need to have keys, phone, wallet etc in your pockets. and you can bring stuff like extra clothes, tubes, all the tools you wanna, maybe camera, DRINKS, maybe some choccy or other stuff to eat, and other friends' stuff of course...