Radial engine, singe engine, fast as a thief, and heavily armed. Liquid cooled engines are vastly more vulnerable, one fragment into the radiator and you, at best, lose the aircraft. Radials can have multiple cylinders shot OFF the engine and stay in the air. 20mm cannon are better than .50 cal, which are in turn better than .30 cal.
In exact terms the F4U-1C Corsair. Four 20mm, cannon, 400+mph, 4,000 pound bomb load. Death on a stick.
Runner ups: P-47D, F6F-5
Close but too late (and likely the best piston powered attack aircraft ever produced) - A-1 (later AD-1) Skyraider.
Best multiple engine: B-26B Block 55. 12x .50 cal forward firing, 2x .50 cal tail stingers, 4,000 pound bomb load. Runner ups: B-25J-NC. Up to 18x .50 cal, 3,000 pound bomb load. Bristol Beaufighter: 4x 20mm cannon, rockets or 500 pound bomb load.
The Mosquito could be fit in here, as it could as a tactical bomber, strategic bomber, night fighter, heavy interceptor, and photo-recon aircraft. Incredible aircraft, arguably the best design in the entire war, certainly the best prior to the introduction of the B-29, which more or less was the first really "modern" bomber. Even then, the most versatile aircraft of the era. Thinking not just out of the box, or even out of the room, but in an entirely different building. Downside is the water cooled engines. Down in the weeds there are lots of potential golden BB flying about.