F4U Corsair, without a doubt. The A6M was a remarkable aircraft during the early war years but the Corsair out-classed it. The only advantage the Zero had over the Corsair was low-speed turning.
The Corsair was faster, more armed, better armoured, had a faster climb, faster dive, faster acceleration, high speed rate of turn, rate of roll, range...and, basically, was just better in every aspect except low speed turning.