My personal favorite is my Fluke T-5 tester.
Measures volts, continuity, and amps to 100.
I've got myriads of hand tools, and my belt is a "Boulder Bag" setup that'll carry 1/2 the van inside, but it's amazing how many service calls I can wipe out by first going in with only a 10 in 1 screwdriver and a tester.
