I just stayed at CSR about two weeks ago with some friends and we all really enjoyed it! Most people here seem to think the Cabanas, particularly Cabana
9b I believe, is the best b/c of its closeness to both the pool and the main building. However, I stayed in Casitas 1 and loved it. I personally prefer the look of the casitas with all its beautiful architecture and fountains to the beachy feel of the Cabanas. Plus, it was about a 2 minute walk from the main building, gift shop, and all the eating areas. However, the walk to the main pool is a bit longer, maybe about 10 minutes, but we never minded.
So it really depends on what you're looking for. If you want a short walk to both the pool and eating areas I would say Cabana 9b, but if you don't mind walking a bit I would say the Casitas, particularly building 1. The rancheros are pretty secluded if you like the southwestern desert feel although some are not too far from the Dig Site.
Good luck and have fun!