1. Least rugged, most well-worn: South Kaibab/Bright Angel loop (see post on mule p**p).
2. Medium-rugged: Boucher-Hermit loop (first night at Boucher Rapids; second day, hike the Tonto Trail to Hermit and camp there)
3. Most-rugged/most solitude: New Hance-Grandview. Note New Hance Trail is not maintained; you must be comfortable with occasionally having to look for cairns marking the route, or scooting down a couple of boulders on your butt. It's still Class 1/a little Class 2 and no exposure. But I would not take a newbie down it.
#2 and #3 are equally gorgeous. I'd choose #3 if I want solitude, but if it's beyond the abilities of anyone in the group I'd choose #2. If you have any true newbies, they will feel a lot more comfortable with #1.