The title says it all.

Which custom maps would you say are a must play?
Which single player maps, custom campaigns and multiplayer maps should everyone have played at least once?

I like Kayland, The Riverland Wars, and Silver Marches. That last one is based on D&D Forgotten Realms.

I hear Xelm Plateau is really good, though I haven't tried it myself.

Lol, shameless plugging for Southern's work in progress. :P