Never thought of a "best time" Rich. I'm not much on snow but do like the look of white snow on the red rocks. Spring and fall are comfy. Summer isn't bad if you aclimatize to the heat as the weather warms up. If you like the great bargains from the roadside stands of Navajo pottery and jewelry there are more set up in the summer. More bikinis in the summer. Fewer people in the cold months. Guess it depends on what is important to you. Canyonlands are always pretty due to the ever changing angle of the sun on the rocks.

Ron