Most fun? I personally don't think that anything else short of full auto can compare. There is absolutely nothing else like that. The senses are on overload after pulling the trigger on a 50. Pure shits and giggles to the max! However, YMMV .
If it weren't for the NFA this may well be a popular home defense gun. Notice how well he controls it in full auto due to the lack of recoil. It would be like spraying a garden hose with a controlled stream of .22lr.