I have it for PS3.
My favourite changes on the situation.
At the beginning, I'm one of those cheapo's who pick scout on 2fort, and just run in to get the flag as fast as I can.
Once the engineers have built sentry guns so I can no longer get in, I wait for a spy to destroy them(or do it myself with a spy) then (since I have pretty bad aim) I use the Pyro.
Near the end of the game, where sudden death is approaching, I'll use my favourite 'the Medic' as I don't have to shoot, I can regenerate, and my team members always need me.
So those are the 3 characters I use the most, but I get the most points with the medic (or scout if its an easy game).