I think Forest would be great.
It's easy to replicate in the Halo engine and easy to not make it look boring either, unlike a desert theme. Though Desert can look good, like shown in Halo 3, it took alot of work and environmental effects to get it not be so boring. ( same with everything else, but this one seems like it would need ALOT of nice stuff for a good atmosphere other then a super pretty sky, and great architecture.) Snow is always cool, but im not much of a fan of it really, and it probally wouldn't look very good with the awesome sky you guys have now.