I haven't been everywhere, I hear Iceland is Great and Some of South America Spectacular.
But if I had to choose it would be between Yosemite NP and Yellowstone. Yellowstone is more varied, But in ultimate spectacle, Yosemite wins and I still belive it is the best NP, even though it has been done.
Personally for less known and photographed, I like The Anza-Borrego Desert
I don't know if I could pick one place. One thing I know though, is that I prefer to photograph landscapes where the sky is at least as interesting as the ground. So it would have to be a place that gets complex photogenic weather. Some of those dessert locations, while beautiful, so often have cloudless skies...
In couldn't agree more about Clouds having immeasurable impact in the quality of a photo..That's why you know what time of year to shoot the desert, which would be now. Some of my best cloud skies have been shot in the desert. If the I don't see the clouds I need, I don't go
Antelope Canyon will be interesting for a few hours but definitely not a DAY. Plus, your camera will not survive a day of sand raining on it.
Zion, Yosemite will be worthy of a full day.. heck, a week each!
Also, Jiu Zhai Gou in Si Chuan China is definitely a few days worth, but since it gets upwards of more than 10k visitors each day, it's reeeeaaallly hard to get a clean shot of anything.
I'm sure there are many places in the Himalayas region worth dying for too!
Some of the most interesting scenes I've shot haven't really been in exotic places. But I would have to say a really cool place to shoot is in the Smokey Mountain national Forest on the boarder of Tennessee and North Carolina. Great scenery and amazing sunsets.