Probably two of the simplest stats are Ks and BBs, or variations of them like K%, K/BB ratio, etc., for both pitchers and batters. Everyone uses them without much of a second thought, probably because they've been around forever. At the same time, Ks and BBs are probably the most subjective stat there is, far more subjective than defensive metrics. Not only do umpires have different strike zones, but how often do they miss calls? Additionally, it has been proven that umpires have biases.