I would go for Deck-17. I have always been a fan of the Decks. This map has seen all the faces of UT and still continues to be hot. I find the gameplay here very balanced. There are two 50 shields, and the placement of the 100 shield is just a genius idea. It's very very risky to try that without proper backup in team games. Also, Sniper Rifle, Shock Rifle, Flak Cannon and the Rocket Launcher are all placed in two different places on the map, allowing a fair game. There is also a Redeemer if anyone is interested.
However, I am also a big fan of Rankin (in any mode of gameplay). Grendelkeep, Campgrounds and Goliath also fall into my favorites, but not as much as the aforementioned two.
Edit: Only maps with a minimum of 8 for upper limit of number of players is given in the poll. If you prefer some other map not listed, please discuss.