Quite honestly, I'd have to say the United States. I've been to 23 or so countries so far, and I have yet to find one with as varied landscape as the US. I will add the caveat that I haven't been to New Zealand yet, and given the scenery in the Lord of the Rings movies, there's something there.
However, think about the varied terrain from Alaska to Oregon to California (northern and southern) to Arizona and New Mexico to Colorado to Montana to Michigan and the Great Lakes to the Louisiana bayou to the Ozarks to the Gulf Coast to the Florida Keys and back up through the Atlantic coast and along the Appalachian Trail all the way to Maine. Oh, and there's Hawaii, too! The sheer size of the country encompasses nearly every climate imaginable, and the varied terrain with two major mountain ranges, two oceans, and many mighty rivers is astounding when you step back and think about it.
I think many people who live in the US don't realize how lucky they are. Having lived overseas for over two years, I can't wait to go home to the States.