a lot of guns are useless if they're very weak and have short range. A hurricane Mk.I has 8x .303 caliber MGs, but this is a light round and has a very short effective range (shooting outside 200 yards is just stupid, because you only get a fraction of the damage and concentration).
If you're asking what's best? Who knows. What's the best compromise? That's a more attainable answer.
6x50cal (or 8x50cal in p47s) or something with 2x20mm is your best bet.
I believe HTC has said that a single 20mm is as powerful as a 3 50cals, and I think 1 50cal is as powerful as 3 .303s (or 7mm). So a single 20mm (when it hits) is the same as 9 .303s. You could take out the 7mm MG ammo on a 109 and the 20mm hub gun would still be more potent than the 8x .303s on a hurricane 2, for example.
So something with 6x (or more) 50cals, p51, f6f, p38 (the 20mm makes up for only having 4 guns), or something with 2+ 20mm cannons, spitfires, fw190, ki61, la5/7, etc.
Planes like the C202 (2x50cal, 2x7mm) and p40b (2x50cal and 4x30cal) and the 109f (1x20mm and 2x7mm) are often considered underpowered, as they have to sit there and pound the target for a long time to get any results.