Mazes are always fun, especially if they have booby traps, such as exploding crates. Race tracks are also fun, especially if you utilize the gravity lifts, which make it more interesting. Also building strongholds is great for the infection game variant, but only if they can eventually be over run. Making a King of the Hill map in which random explosives drop from the sky is hilarious, my friends and I played that for months. Such good times.
But with any good map, there has to be a good game type. Trying to think of a specific game type in mind when you are designing a map. Try not to make anything too bad or too good, try to make the map balanced for each team/player.
