BK- Spiral Mountain. This was the beginning. It promised so much, and delivered. I remember playing the demo of Spiral Mountain in Wal-Mart when I was 10 and thinking, "Wow, I could just explore!" And a fire was kindled. The music was lovely, the scenery was beautiful, and dialogue with Bottles was very funny.
BT- This one was tough, but I guess I'll go with Grunty Industries. It was massive for a five story building. There were tons of nooks and crannies to dig into. Heck, even getting inside was an adventure.
PU- This is perhaps a bit of an obvious idea with Pagies opening worlds, but I'd love to see a Mother Goose world. It could be full of Fairy Tale and Nursery Rhyme characters (in the public domain). Expanding the world could unlock various areas. First there could be a village with an old lady in a shoe, Mother Hubbard, Jack (who is both nimble, and is to sell a cow). Expanding once could grow Jack's beanstalk, letting you climb to a giant's lair. Twice could unlock an encounter with a troll guarding a bridge leading into a forest populated with Goldilocks and/or Red Riding Hood and company, for example.