Charlie. Because it's hard. Because it's unfair. Because you have to work together. Because you need to communicate. Because there are very few kills as enjoyable as when you finally get off the beach and get to revenge your self on a sniper who's killed you 8 times in a row. Because there is stress when you're the only one left at big-gun and the allies are swarming you from the front & you can hear the elevator coming up. Because there is a time limit, and you either win or lose.
Then Donner - the classic remake of Thunder from 1.3.
Kalt, hands down. I love it because, it was the first eGO server I played on, and because I know it so well. I also like short maps that the fights last forever on (when there is enough strategy being used)