From a purely physiological point of view, the spinning back kick uses all the biggest muscles, plus centrifugal force and linear momentum.
But, from a combat perspective, and, keeping the strikes "legal", I'd say knees.
To the liver from side back control (GSP to Serra)
Knees to the head from the muay thai clinch (Silva vs Rich)
jumping spring knee to sitting opponent (rashad/silva to tito/chael)
Granted, a lot of the time when those strikes are landing the opponent is already hurt from a previous strike, but those are certainly "finishing moves".