For me it has to be the Advanced Marauder - in my noobie hands it's not exactly a killing machine against anything but poorly armoured folk, but it wears the strongest down and is a bugger to hit..it also is great distraction if you're trying to protect a near-dead team-mate.
The zap attack seems more fun than terribly effective, though I've noticed that you have a bunch of grouped turrets, it seems to wear and chain a bit..though the amount of bouncing around needed to avoid being ripped apart, whilst still staying near enough is a bit too hard atm.
The last thing is the amount of noise that it makes when getting hit...on one side thats a con, but quite good if you want to feign and pull off people from a group i.e. wait for a couple of squeals (whilst still being 100+ hps) run off, wait around the corner..and it's spread-out crunchy humie time!

The Advanced Marauder - yours for a bargain 3 Kills.
edit: Howie - the snipe will work with middle button..its quite strong, but drops off annoying quick