I started off with heavy because he was a simple class to use and it allowed me to get familiar wit how the game operated. Then I moved over to medic because it let me follow other competent players around and learn their strategies. I didn't end up 'maining' either of them though. On defense, if we've got at least one other engineer to watch my back, I'll play engineer. Every time one of your teammates jumps from spawn to the front lines through one of your teleporters, you know you've done your team good. I used to play a lot of Pyro too, but it seems like people are a little smarter about spreading out now. I'm also partial to Demoman, because the only thing more satisfying then destroying 3 enemies with one stickybomb explosion is firing a random grenade into the battlefield as you turn around to grab some ammo and killing someone you weren't even aiming at.