Rocky Mountain may very well be may favorite place to visit. This probably is because I used to live near it and would travel up to the park and do my homework on a picnic table while I was in Graduate School.
The park has some of the best of what Colorado has to offer: abundant wildlife, mountain scenery, scary roads crossing high mountain passes, fall foliage and hiking to last a lifetime. This is also the only park I've been able to visit in all four seasons.
Gore Range Overlook
Colorado River (near the source) Bear Lake
Aspen Trees in the Fall Alberta Falls Chipmunk
View from Trail Ridge Rd. Morraine Park