The taiga biome is my favorite, but the mushroom and flower forest ones can be really fun too. I tend to build on or inside mountains!
I personally like taking over villages from the plains biome, I make a fence perimeter across all the house and start a little city. It's really fun!
I cannot really pick any one over the others that I can say that I really like the most. But, when I did read the mushroom above that brought back some good memories.