Medic has always been my favorite since I started the game. He's the ultimate team player, even if he's a crackpot. Since I play on pubs a lot I'm usually the medic since NOONE wants to be him, the trigger-happy gung-ho cavemen. There's literally no better feeling than bulldozering through a seemingly impenetrable defense without taking a single scratch with the Medigun, or watching your buddy pop the entire enemy team like over-ripe cherries with your kritzkrieg. Then again, you're heavily reliant on the ability of your team, which can lead to some incredibly frustrating moments when the enemy team is wrecking your entire idiotic team and you can't do much with your piddly-ass needle gun, or when your buddy ignores your shouts of "I'm fully charged! Go for it!"
On the OTHER other hand, there's few things more hilarious than charging at someone and lopping their head off with the bonesaw. I can't even count the amount of kills I've gained from charging straight at someone who expected me to flee (pyro kills like this are especially hilarious) and taking them out with a single 195 damage smack in the face (medic gains crit chance based on his heal rate and melee attacks have high crit chance anyhow). "Oops! That wasn't medicine!"
I should also mention I have an intense hatred for the sniper (camping asshole). Maybe that's just because I'm shit at playing him. I'm also reserving a V.I.P circle of hell for people who don't know how to spy-check or even notice what's happening around them (one game we would be flanked by the same fucking pyro and spy over and over, and noone ever thought to look behind them, even when I called them out, ever...)