Really? In my opinion the Black Wool was the hardest!
I read other people's comment that there's a Santa Boss. I'm gonna guess that it's on the Black Wool and it didn't spawn for me. The black wool area is pretty though, but it isn't too big like the other areas. Many of the spawner on the diamond area doesn't spawn too much, possible because it's too bright? (I don't remember if there's glowstone in there or not).