I often end up being the medic, as people don't want to play it, and it's almost always the most useful class, and I like helping the team. I'm often a medic in TF2, and in every MMO I've played, I've had a healer as an alt, if not as a main.
If I'm not the medic, or a medic class is not available, I go for some general support kind of guy. Suppressive damage, general "hold the line!" business, while the other, more fraghungry players can get the kills.
I almost never play solo classes, who don't directly help team members, such as sniper or spy in TF2. Of course these classes aren't useless, but they're very "lone wolf", and focus on killing as many enemies as they can, instead of working towards objectives.