Pretty tough to pick just one! But I will submit the humble but incredibly versatile CHP-170 cutters. It's a small pair of flush cutters, with about an inch of cutting surface and a slight angle.
The nominal purpose is for cutting leads of through hole components on a circuit board, but it's useful for pretty much any application where you need to cut something small and hard. (makes a great toenail clipper, for instance, lol).
