I am not a big biker but I hear that Montana is a good place for that.

Well, if you expand the definition of America to include Canada I'd recommend checking out Whistler in BC, Canada. It's near Vancouver and the downhill is awesome!

I have heard a lot of good things about South Lake Tahoe, California. Apparently there are a lot of good trails there; several that are on the "Top 100 in America" list. Tahoe is also really good for resorts and general hanging out. It's at elevation though.

I'm not sure about the best place - but I know there are some amazing trails in the mountains around the Seattle area!

Probably Colorado. Although there are lots of amazing places to go in the Rockies.... My cousin moved to Durango, CO for the mountain biking. I live in Washington State (pretty near Mt. Rainer) and there are some good places around here. The Pacific Northwest has a pretty good "bike" culture.