Medic. There's so few of us I never feel like I'm unnecessary.
However, I do feel a bit unappreciated; I tend to end up on servers where everyone begs for healing but refuses to protect the only guy keeping them alive, only concerned for their kill-count (BEING ALIVE LETS YOU KILL MORE!). Just as bad, the scouts that steal the teleporter, meaning I've got to wait for ages to get to the front line where I'm needed.
This isn't everyone, obviously. I have met some really nice people that know how to respect Herr Doktor, and when you find the right team-mate, my kill assists shoot me up to near the top of the scoreboard.
People, treat your medics right. We're a scarce resource.
And scouts; don't scream medic and run/double jump away from us before we have a chance to turn around. We don't like you.