Favorite? Oh man... It's a tie between RidgeWar and Keep Off The Grass. In addition to how beautiful these maps are, they're also quite unpredictable. Every match is different, even if people have caught on to certain strategies (i.e., blizting the Allied Power Plant on KOTG), it just makes countering those easier and opening new avenues.
Least favorite is PacificThreat. Every match, it seems, ends with the Allied leading a naval assault. Haven't seen a submarine here in a long time. It's just dull. Removing land vehicles has taken out a major element of this map, honestly.
EDIT: Retracting this statement entirely as I just played a match with a Soviet victory. I still don't like the map, but eh. Sorry if that offends anyone.
I almost put CI for both, funny enough. I like the map, but after some recent experiences, I harbor a special hatred for it.