If you plan on going and have the time, visit all of the mighty 5: Arches, Canyonlands, Bryce, Zion, Capitol Reef. They are all unique and different. If you don't have as much time, I suggest doing Arches, Zion, and Bryce, as I personally think they are the best out of the 5. If your schedule is really tight, go Zion and Bryce, as they are relatively close together.

Or if you want to make it an adventure, have the time, and don't mind the long drive, start from Salt Lake, go Arches, Canyonlands, Capitol Reef, Bryce, Zion, then head to Page Arizona for Antelope Canyon + Horseshoe Bend, loop to Grand Canyon and back to Salt Lake.