so.. im first to say this.. ocean biomes.. it's pretty much the hardest biome to start in and to expand in, I love building on the ocean floor with nice windows and glass domes, lighting it up with glowstone and jack-o-lanterns. Underwater building gives a nice challenge and if lit up well it's pretty nice. I'm planning in getting something like mo's creatures to get some stuff swimming around my place :-)
oh yea.. apart from that I really like the jungle biome too. because: 1. kitty cats, they are might cute. 2. lots of wood 3. nice high trees packed up for epic treehouses(which I want to try out sometime)
My favourite Biome is by far the Jungle Biome. I just love the tall and thick trees, the vines, the ocelots, the colours, the sometimes little ponds and most importantly, and awesome chance to make a good Treehouse.
Extreme Hills. They have epic formations, especially when two mountains' tops are connected, but their mid-section and bottom are not, or when almost the entire mountain is hollowed out because of the lake underneath it, so it's like this giant cave opening. Unfortunately, those mountain-caves never go anywhere. D: