We've been to Upper, Lower, and Canyon X. We go in March, which helps with crowds. Canyon X was just two of us and the guide, which was special, but Lower is my favorite. My advice is to get to the canyon as soon as it opens, buy the photographers pass, Go down through the canyon as fast as you can make yourself go, and work back up so that you meet the tour groups. You will probably have quite a period of time before the first group comes through, and then they will pass you relatively quickly.
While we were there one of the guides was carrying a wooden flute. We talked him into playing a little bit - Magical!