What class do you usually pick up on games ?

Monster

Swagger

Whatever's stealthy and uses daggers, if given the opportunity.

Warrior mostly.

I always start trying to do something different but I always end up being an assassin (sneaky with knives and bows)

Healer or some rogue/assassin like class.

When I play games with class selection, I usually pick either some sneaky thief-like type (assassin, rogue, thief, ninja... whatever the game has) or some sort of evil priest/ wizard >:)