I-80 has the easiest grades but is by far the least interesting.
I-70 out of Denver is a pretty good pull and can be pretty busy at certain times. But I-70 has good views and a variety of places to stop along the way.
US 40 west from Steamboat becomes less scenic as the miles pass. But the stops mentioned in and around Vernal are worth the stop. 
Busskippers route is quite windy but very scenic. Between ghost towns, rafting, fishing, art shops, hiking, etc., one can spend a fair amount of time (days, weeks) between Leadville and Salida. Once over Monarch Pass, camp somewhere around Gunnison and drive the Jeep up past Crested Butte to Gothic and beyond. In late June through early August, this is a spectacular area to see miles of wildflowers. On US 50 after Blue Mesa Reservoir, keep an eye out for Cimarron. Turn in and park your rig. Take the jeep up the two or three miles to the end of the road, where you'll be in the bottom of the Black Canyon of the Gunnison. It's a paved road but the turn around at the end may be a little tight for your rig, depending on where other vehicles are parked.
One alternative to his route from Boulder is to take CO 14 to US 40 and east to CO 9 and then CO 91 south to Leadville, etc. This avoids the pull up to the Eisenhower Tunnel. 
If you don't want to go north on CO 133 to Glenwood Spgs and Aspen from Black Canyon (some great scenery up here though), just continue on US 50 to Grand Junction and connect back with I-70 there. While not the size or scope of the scenery around Moab, Colorado National Monument is worth a day to drive and take some of the short walks to scenic pullouts. If you do go up to GWS, head west on I-70 to Rifle and catch CO 13 north and then west on CO 64 through Rangely to Dinosaur if you want to get to Vernal that way.
Sorry this is rambling but post more questions if you need.