"Best bomber killer" in terms of guns only??? Easy. That would be the Me410 on TWO accounts.
First, the Me410's 50mm cannon truly is a 1 hit marvel. Once it connects with a bomber there is no question the bomber is going down.
Second, the Me410's dual 30mm delivers monstrous hitting power, at a high rate of fire, and at a very usable trajectory. Remember, the 30mm's on the Me410 are NOT the same as on the 262, 110, 190, or 109. The Me410's 30mm's are the Mk 103 30mmX184 and not the typical 30mmX90 most everyone has a love/hate relationship with found on the other German fighters. The Me410 when armed with the 30mm's is almost as deadly as the 50mm, the rate of fire is much higher and there are 6+ rounds hitting the target for every 1 round of 50mm. It depends on the pilot and technique used. The Me410's Mk 103 30mm's are much easier to use, they have a trajectory far more similar to the 20mm's than the Mk 108 30mm's. Try them and enjoy!
Third, I think it is hard to beat any quad 20mm armed fighter in terms of shooting down a bomber. Sure, the Mk 108 30mm found on many of the German aircraft offer massive hitting power, but they are not the easiest to gauge in terms of trajectory. I'll take the quad 20mmon the Fw190A-8 on most occasions. Oh, and don't forget the Tempest, Typhoon, F4U-1C, or even the Sea Hurricane or Hurricane IIC.
Vs tanks I believe the IL-2 has the advantage for one simple reason: ammo capacity. However, I think much like the "bomber killer" scenario there is more to think about than just 1 thing. The IL-2 ranks at the bottom in a couple of very important categories for me. The Hurri IID is faster, climbs better, rolls better, and has a far better view of the battlefield than the IL-2. The Stuka offers nothing more than the IL-2 other than a better view of the battlefield. However, it is far more gratifying to land kills in the Hurri IID and Stuka G-2 than the IL-2 as well.
The guns are all very similar. The IL-2 does seem to take a back seat in accuracy, too. But with 50 rounds per gun it doesn't matter. It is the ammo amount that really only separates them, the AP ability of each seems to be very similar.
For killing ack, keep it mind all it take is a single CANNON round in or near the auto ack/manned ack and the job is done. Anything AP needs a direct hit and that includes the US fighters with .50 cals. I can de-ack a small airfield using less than 20 rounds of 20mm from a 109. One shot per gun emplacement. Aircraft armed with the 37mm can do it as well, but they have far less ammo and a worse trajectory.
Bomber defensive armament? The B29 hands down. Then the B24 (more guns active more often than the B17), then the B17, then the B26. The G4M would be good as well, its rear 20mm is nice but I don't use it enough to be efficient with it.
