PM-63 is no doubt my #1 favorite to shoot.
Second favorite is either the Colt model 635 or FN P90; really nice guns so I'm stumped between the two.
Third is probably an UZI (with the wood stock, I wouldn't want put my cheek on the folding stock during full auto lol).
Fourth is gonna be the MP5a2, I've tried it a few times from different owners, but it never really "amazed" me like what the hundreds of people tell me; it's a great gun, don't get me wrong, I just think it really isn't as great as people make it out to be, especially with the price tag it carries.
I haven't shot anything else full auto besides those, it's a bit hard finding machine guns in CT lol, almost all of the ones I've shot were when I was out at Pennsylvania, Colorado, and Georgia.