Rawah Wilderness west of Fort Collins. I haven't been there for a long time, but it was one of my favorites. The NeverSummer Wilderness just outside the NW corner of Rocky Mountain National Park is spectacular, although more rugged than the Rawah. There is also the Mt. Zirkel Wilderness east of Steamboat Springs. These all get you out of the NPS bureaucracy, but into equally beautiful scenery.
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey