cp_dustbowl. Never play anything else. The game is a beauty on this map. CTF is too derivative. Arena, yawn. The CART maps are worth playing. The cart and capture point maps (defense/offense) are amazing because they force the teams to be TEAMS. You have to push the cart to win. You have to push forward to win.
If you're wondering which class is hands down the single most powerful look to the Demoman.
Can any other class recognize that their bullet is a CRIT in mid-flight and detonate it at will? I think not. Hands down.