Personally, I find it be the Berserker. The reason I say this is because of the health regeneration when you activate the special ability. This comes in hand, especially once you get get short fuse up to level 5, because you can use Berserk every 30 sec. This is just my own personal opinion. You'll have to try each character out and find out which one you like best. ;) Have fun!
Bad question. Ther is no best class, each of them is unique and each of them requieres different playstyle. Personally, I like Siren and wouldn´t want to play any other class because I love using SMG and rushing into battles.