Lets go by role. Bomber: The B-52 for its longevity, versatility, capacity, deterrence, fair speed and high fear factor. The B-1 for speed, and B-2 for stealth and capability. Honorable mention to the Russian Bear and the B-17. Fighter: in its day, the P-51 rules for firepower, range, fear factor, speed, and agility. honorable mention to the ME-109 and Mitsubishi Zero. Modern: likely the F16 for its success in all roles and honorable mention to the Su-27 and Raptor (awesome but too few produced to be fear-inducing). The military channel has whole programs on this question, but break it down to helos, fighters, bombers and strike.
There's tons of different criteria, but for my money - the B-52 has been in service since 1952, has dropped more tonnage of virtually any type of munitions ever invented for a bomber (including nuclear cruise missiles), and is still in service for the foreseeable future. The Big Buff ain't pretty, but the expression 'Most Bang for the Buck' fits it perfectly.
The one that brought my father home all the years he was in the service. Your answer your looking for is the one that fits it's situation the best could be a chopper or her their different for each of thirty jobs.