The Meaning of Life, the Universe, and Everything.
My favorite has to be forests next to an ocean, not a jungle or one of those snow forests but on of the original forests. there is something immensely satisfying about building a wooden house on the shores of the ocean with a forest behind you, diamonds below you and an ender dragon in another dimension planing world wide destruction...
Mushroom biome. It's aesthetically awful to build in, but after moving in to a neighbouring biome —or the mainland if the mushroom biome is in an island— it's very convenient to have a place where to herd mooshrooms from. Even more so when we get the next update that includes leads.