EDIT: A lot of time has passed since this post, and a lot of new hats have come out. To see my new favorite hat, check this out.
A while ago, I decided that the Top Hat was the best hat in the game. When trading comes out, I'll trade any of my existing hats for one. There's a mod that some servers have that allow you to wear hats that you haven't earned, and on any class.
This allowed me to test my theory: Is the Top Hat not just the best hat for one class, is it the best hat for every class? (The answer: Yes)
PS. I lied about the Heavy, the Top Hat actually clips through his forehead and looks kinda stupid.