So many options... If you are looking to make time - stay on the Interstate. CO-50 is a gorgeous drive but there is Monarch pass - great road but it is steep. CO-160 (into Pagosa Springs) is beautiful and road is great - Wolf Creek pass is steep coming down (less so going up - given your direction of travel).
If you prefer Utah (more desert, less mountains), SR 191 down thru Moab, Blanding, Chinle is a good road.
SR-60 between Soccoro, NM and PHX is very scenic but you do have the Snake River canyon to get down/up.
Flaming Gorge is nice if you want to cross west further north. I-80 and then 191 south. Very pretty. One pass (that I recall) just south of Flaming Gorge and then the descent into Vernal.
Assuming you have an engine brake of some sort, coming down the passes is no problem - let it do the work. I've taken all of the above roads in my 40' Dutch Star (370 HP) towing my Jeep. Going up - find a nice lower gear, throttle to about 1500-1600 and putt on up.