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...