Here's what I've needed/used over the years. I have all this stuff in my field box, and use it at the field and at home for setup, building, fixing, etc.

1) Metric allen wrench set
2) Standard allen wrench set
3) Large & small phillips screwdrivers
4) Large & small flat head screwdrivers
5) Micro or "Jeweller's" screwdriver sets, phillips and flat head (servo screws, idle needle adjust, etc.)
6) Electrical wire cutters
7) Needle nose pliers
8) Standard and Metric nut drivers and/or socket set
9) Crescent Wrench (large enough for your largest prop nuts)
10) X-Acto knife

Optional stuff:
1) Glow plug wrench (8mm socket works too)
2) Z-bender for wire push rods
3) Small bolt cutters for wire push rods
4) Tachometer
5) Small ruler (measure throws)
6) Caliper (measure shaft & screw diameters, etc.)