Hmm, tough one.  I kind of like all the seasons, for different reasons.  I guess since I have to pick one, it would have to be fall.  I know, I know, it's fall right now, so it's a bit biased.  But here's why I like it.

Balloon Fiesta October 2009

Balloon Fiesta October 2010

4th of July Canyon

Rio Grande Bosque, near Alameda

Alameda Bosque

North Bull Basin, Gila National Forest