I prefer driller, but rotate regularly to fill (and promote). I feel like it's the most versatile class in the game.
- Bugs? All 3 weapons do AoE very well and bring extra utility (robots on fire, enemies frozen, bugs slowed, etc.)
- Stuff on the wall? EPC + Thin Containment Field
- Gotta get somewhere? Drills
- Teammates? C4
That said, they're all fun in their own way.
Edited for formatting
Were it so easy...