Depends. If I want to fuçk around and be an annoying little shit i'll go Gunslinger Engie or Scout. If there are too many fuçking scouts on the enemy team i'll go Demo or Pyro. If there's not enough power classes to push through a chokepoint because everyone's playing Spy for some goddamm reason i'll pick Soldier or Heavy. If I want to be purely supportive i'll run Flare gun Pyro, Sydney Sleeper Sniper, or Wrench Engie. If I want to die inside i'll pick Spy. Simple.