The h8k2 has impressive stats but these were often accomplished with reduced capability in other areas like armor. It was a pretty good looker for a flying boat.
By far, the one with the biggest impact to the war was the PBY.
The H8K is my vote, it had good combat records, good speed, good armament, and was notoriously hard to shoot down. My vote is strictly on performancce, as obviously the Short Sunderland and Catalina played a bigger role in the war effort.
I agree, the H8K had high performance and excellent armament. Unlike many Japanese warplanes of the period, the H8K also was well-protected with armour and fire-extinguishers for the fuel tanks. IIRC, the H8K's fuel tanks also had a bilge system that allowed leaking fuel from punctured tanks to move into intact tanks.
Gentlemen , for me it has to be the Sunderland.
She has her faults - greatest problem was that she was underpowered but the aircraft was loved by her crews to them she was second to none.
The attached photos show 422 RCAF Sunderlands at Castle Archdale on Lough Erne , N.Ireland in 1944.