As you say single handedly I'm guessing you're ruling out bomber pilots and such.

Possibly Sgt. Dillard Johnson - a US tank commander and sniper who got 2746 'kills' in the second Gulf War.

If you're really talking about single handedness (tank crews are, after all , a team) then you're probably talking about snipers - the deadliest being Finnish Simo Häyhä with 705 'kills' in the Soviet-Finland 'Winter' war of 1939-40.