Yeah, Nightstalkers cool and all, but space cowboys man. SPACE COWBOYS!! Gunslinger with the last word is always a good time. Maybe not the most efficient build, but who cares when you're having that much fun.

They're all quite good to be honest. I like Gunslinger the most though. I just prefer it's perk tree. If you're looking at pvp then Nightstalker. It's super is actually one of the most unbalanced in the game due to its extreme versatility. It can one-shot and during animation has high damage resistance (I've seen it take a Nova Bomb) and the shots track. Nightstalker kinda has 2 grenades due to smoke too.

Nightstalker