Very hard decision. Recently I've really been enjoying firebug, as I really like the touch-and-burn mechanic, but long-term I have to say Sharpshooter is my favourite. As long as you can get to them before someone starts angering them, I can support the team by taking out just about all of the scrakes and fleshpounds. Really takes proper teamwork and cooperation though. Plenty of different pistol combos are sufficient to take out the rest of the baddies.
I also have to throw my love in for beserker- It's fun being in the middle of everything, adrenaline rush and improvising everything. I play katana of course, then get a dwarfs axe for independent husks and sirens, rushing scrakes (cuz you can knock them back) and of course fleshpounds. I'm usually the crazy beserker that rushes the fleshpound with my axe after all my armour's gone, hey, it works sometimes.