My favorite biome is certainly the desert of Harad. I like the lonely atmosphere of this region and the relaxed personality of the nomads who mind more to trade than the war. The biome has a very good style with its rich diversity of variant and mostly the sandstorm instead rain and the ruined fortresses buried in the sand and old pyramids. There's alot of treasure in these abandoned structures either, with traps and wraiths to deal with to get a bit of challenge and adventure feeling. And the desert of the Harad is one of the hugest biome of the mod: You can have fun for many hours in this biome. And it's quite peaceful! There's no invasion, the only enemies you find in the desert are scorpions and the scorching sun.
My least liked biome is Ithilien. Even if it's a very pretty biome, it's hard to stay long in it without dying because invasions are damn common. As a good player, you are permanently harassed by orcs and wargs at night and say welcome almost every day to warg bombardiers and olog-hai during invasions. And as an evil player, say hello to the swarm of rangers. Personally, I always avert this biome if it's possible. And except to hire Ithilien rangers, to get lebethron wood or asphodel, I never go there if it's not just for thrashing.