I like to play the Medic, Soldier, and Scout, though I'll usually play someone else if it looks like my team needs an extra Heavy Weapons Guy/Pyro/Engineer, etc.
My least used classes are the Demoman, Sniper, and Spy. I have an unfortunate case of Storm Trooper Syndrome while playing as the Sniper. As for the Demoman and Spy, I just haven't gotten around to playing them much.
Just a while ago, I was playing on Granary and nearly my whole team (myself included) Scout rushed point A and easily took it. Scouts are like the Zerglings of TF2.
Like Growlers at Sea said, the Medic is only interesting to play if your team is doing a big push and you have a lot of teammates at the front lines. That way you can stay behind, heal a lot of people, and maybe bonesaw the occasional spy.
The Scout suffers from the opposite problem. They're only useful for light skirmishes and capturing relatively unguarded points. During the huge battles at the beginning of matches, there's just way too many rockets/bullets for the scout to dodge, unless he can sneak around using some alternate route (which doesn't always work, or in some stages, isn't an option) Sentry guns tear them apart as well.
The one thing that annoys me the most about the game is Critical hits. Is there some specific way to pull them off, or is it just some random chance thing?