1st: Soldier, so I don't have to aim (unless I'm sentry busting, then I gotta use the direct hit).
2nd: Engineer (typically on red) so I can use Eureka Effect to get behind blu team by putting teleporters in stupid but effectively hidden spots and spawncamp.
3rd: Sniper. Even though I said I can't aim, when the map is full of straight corridors and the server is full of oblivious players, then I can just use the piss rifle or stock and get some easy kills.
Wow looking back at this I might as well make 4th be a lime scout, huh. :^)