I typically play an agressive defense Demoman. If you can get used to the projectile nature of the pipebombs it can be a very powerful class as you can bombard an area from around the corner. The remote-detonate pretty much guarantees that the person who touches my intel first dies.
Nickname was a monster Pyro before the achievements were released and Kuros is full of win when using a Soldier. Kalia is a decent Medic.
All classes are good when used correctly and they all have benefits. Tbh, the game is very well balanced to contain so much Valve code.