I picked Spy, but really it is a breakdown for me:
Spy vs inexperienced players: My. God. You will rack up kills faster then you can believe, and on a good day, watching the enemy team beat one another to death while you come up behind them.....joy.
Sniper vs team with bad or no spys: Just rack up head shots in relative safety. Dodge bazooka shots.
Demo vs ENGINEERS EVERYWHERE: The team with 8 turrets turtled around the final cap? RAIN FIRE UPON THEE.
Heavy vs MY TEAM WHY WON'T YOU PLAY HEAVY????
Medic vs MY TEAM WHY WON'T YOU PLAY MEDIC????
Soldier for fun.
Pyro when the enemy team angers me and they must burn.
Engineer is probably my favorite class, I love to run the mini and go on the offense, especially on payload.
And I don't like scouts.

Sorry.