It's really hard to limit it down to just five. The results may be different tomorrow...or even an hour from now. Not in any particular order:
Yellowstone National Park -- beautiful and unique in the world. Yes, it does get crowded during the summer.
St. Louis City Museum -- so much fun. It also can get busy at times.
Wrangell/St Elias National Park -- There are many beautiful places I got to see in Alaska, but this was perhaps the top of the list for me. The Brooks Range along the Dalton Highway, the Turnagain arm, and the general area around Stewart are also exceptional.
Letchworth State Park, New York -- This could very easily be a national park. Lovely waterfalls and hiking and so forth. (I also very much enjoyed Watkin's Glen; it was somewhat of a toss-up between the two.)
Ice Fields Parkway in Banff and Jasper National Parks (Alberta, CA) -- Stunningly beautiful.