145, placerville to cortez, loved it, quite scenic
160, cortez to del norte, liked it, quite a bit more traffic at times. mesa verde is a worthwhile detour, yes it is uphill to the CG, and a bit of a ride from the CG to the ruins, count on a day for a loop to the ruins from and back to the CG.
50, poncha spring to montrose, liked it, can have strong winds blowing from the West around Blue Mesa Reservoir.
Ok, I at least like about everything!
I don't recall any of these inciting much fear, other than a section with CDOT dump trucks roaring by at 50mph on the gravel when we couldn't keep up with the pace vehicle in a construction zone.
550 from durango to ridgeway is a worthy alternative to 145.
If you don't mind some 4wd road you can get from silverton over cinnamon pass to lake city (not for the faint of heart, count on some pushing), then take 149 N. See http://tsteven4.qwestoffice.net/gallery/index.html?albumid=5361805162126665921&si=1
which also covers some of 160, 550, 145.
From Blue Mesa Reservoir you can take 92 NW, 133 NE, 82 SE through Aspen over Independence Pass, 24 N, 91N to I70, bike path E to 6 over Loveland Pass, I70 corridor E mostly on frontage roads. Yes, I am not one for straight lines. Don't know how you would get through Denver if you are heading east though. If you want to go north from around Idaho Springs watch out for Black Hawk, where cycling is against the law!
Lovely riding in western Colorado, I would highly recommend it. Be prepared for cold mornings (~40 but could be a bit colder) in the mountains in summer.
There is a CDOT map with shoulder ratings and traffic volumes, http://www.coloradodot.info/programs/bikeped/colorado-bicycling-maps/scenicbywaysbikemap.pdf/at_download/file
For a hard copy of the Colorado Scenic Byways & Bicycle Map, send your name and mailing address to email@example.com