Mystery Canyon in Southern Utah...
This is, by far, my favorite slot canyon I've done so far. Only a few people are allowed through each day so you'll need to wake up early to get to the visitor center to get your permits.
You'll encounter some good rappels (13 in total and up to about 150' I believe, so you'll need two ropes), a couple of the rappels are over waterfalls, you'll also get to see indian ruins and gorgeous scenery.