My favorite class has changed a bit with time. When TF2 was in beta, the demo man was unbelievably powerful and people seemed to forget about the sticky bombs. Even though they “nerfed” the demoman (twice), I still play as demo a lot.
Recently, I’ve been one of those people going for achievements. I like to think that if I’m #1 or 2 on the team then it’s OK and I’m not just farming for new guns.
Those are my favorites, however a skilled player can lead the team with any class. Even a Medic or Engineer can consistently dominate.