I'll bite...
California. Why? We have more in one state than any other. Other places are really great; I have been to a number states; West, midwest, north-eastern. Hawaii, US Virgin Islands, western Mexico, BC, Alberta. Most are really nice, and have lots to offer.
But, nowhere else has all California has. We have ocean, from warm beaches in the south, to rocky cold shorelines to the north. From the big city and ocean in San Francisco, you can go to the interior valleys and foothills. From there to high mountians, alpine lakes, big trees, and in to the desert.
There's big cities, like San Francisco, the greater Los Angeles basin, San Diego. Then there's neat little places, like Cool, El Dorado, James Town, Bodie. There's tall peaks, and the lowest place on earth in Death Valley.
Alberta Canada is beautiful(Banff, Kootenay, Jasper National Parks)! Wyoming is amazing. Vermont in fall is stunning. The beach in Cabo San Lucas, BCS Mexico is great.
California though, has everything. Aspens quaking. Redwoods in the mist. Alpine lakes. Desert vistas. City life. Rural life. Rivers, lakes. National parks; Yosemite, Death Valley. Coastal towns on beautiful beaches and rocky vistas. Awesome winter skiing. US Olympic trails(In Sacramento right now, track and field)
It's hard to beat for an all-around adventure Other places can immerse you in thier granduer, but not many can have you to another type of climate or terrain type like here. That's one reason so many people are here, and so many move here.