Alaska. I've only been once, for a couple of weeks. I really want to go back, probably several more times, because I feel I've just scratched the surface — but what I've seen so far is spectacular.
In the lower 48, I'd have to go with Colorado. So many great mountains and again, I've seen only a handful so far. I really want to see more of California and Washington, too.
I also love North Carolina. I went to college there, and played A LOT (climbing, hiking, backpacking, etc) in the mountains there. The beaches are nice, too. Calabash has the best seafood in the entire world.