Medic because it's so overpowered. A machine gun that's even accurate at long range, infinite heals, and 50 points for defibing dead teammates. Talk about racking up on the points.
Honestly, I love the medic class in general. That's all I play now in Team Fortress 2, my favorite class in both Bad Company 2 and Battlefield 2, and even my character in Dragon Age Origins is a healer. I like all of the classes in Bad Company 2, just medic is my favorite. I mean, I love sniping, using the grenade launcher, and using a rocket launcher, but the medic is just my favorite hands down.
It all depends on the situation, as well. I mean, if your team is being rushed by tanks and helicopters, the engineer would obviously be your best choice. Sometimes I just want to rush in and shoot everybody up, then I would play engineer. And sometimes I just want to relax and gain a bunch of points, then I would play recon. Depends on the situation.