I absolutely love the surface hallow biome. If it isn't where my central base is, I at least have to have a music box there with the theme recorded on it. I feel that, despite the enemies that come with the hallow, it just feels more relaxing and peaceful... like a magical forest or something. I really like pearl-wood and pearl-blocks too.
But my second favorite is generally a smaller one, that would be the ocean. I like the music there too, and I have built many houses there before. Just wish they were bigger! I would love a connected world, with larger oceans. Anyways...
My favorite is the Underworld/Hell. It's the last place you go before Hardmode, and can get many of the best pre-hardmode items from hellstone. Once hardmode begins, it's one of the safer areas of the world, and once you defeat a mech boss, you get even more action in Hell, along with a few extra drops (such as Fire Feather for Flame Wings).
I like the jungle I set up shop there in 1.2 and don't regret it the environment is so distinct with its eneemies, thick growth, theame, and difficulty there though drawbacks, notably bloodmoons can cause trouble at times when an angry trapper spawns inside. Additionally its always annoying to make sure you quarantine is working properly as the evil biomes love to eat at your jungle.