The MK108 is the best "Large" cannon in game. Yes its lacking in terms of Ballistics and Muzzle velocity, but it has one attribute that all the other big guns don't have and that's a high rate of fire. It is also light enough to be mounted on single engine fighters without seeing a significant drop in performance.
It is not the best gun in the game, I don't think it would make the top 3. It's simply to hard to aim for your average stick. Guys like Grizz Anarchy and the other expert tator chuckers the expectation not the rule.