Estes Park – meh. Not a bad little place, but the town itself doesn’t get anyone excited really. I wasn’t sure of what activities there really were to do either. It caters for lots of casual tourists; Rocky Mt National Park (which it’s the base for) is also pretty restrictive. You can’t ride a bike off-road anywhere in it, for instance, and I didn’t see much stuff to do other than drive around and gawp at wildlife or climb big peaks.
Colorado is a big ole place mind, I only know that part of it. Wouldn’t go back. Seems like there’s more pressure on land use though generally, whereas in Utah no-one seems to be trying to grow or farm anything or sell houses so outdoorsy people seem to be able to do as they please.
IMO Yellowstone is just wow.
Yellowstone is wow as a tourist yes, but it’s not clear how much exploring you can actually do. Or even how safe it is – steam vents and sulphurous bits seem to come and go so trails can become unsafe over time. Grand Teton next door could be cooler – very dramatic. Never went there though so don’t know what activity opportunities there are.