Area around Rapid, Northern Colorado and C-springs - all areas with GREAT roads. Getting from N. CO to C-springs - listen to someone else. I know there're godd rides there - but that's outside my circle.
Problem is what's in between Rapid and N. CO. You've a long chunk-o-road to cover from SD to the Ft. Collins area - get it over with. More enjoyable route I'd run is:
Rapid City, SD (do the needles - take the time - plan an extra day if you have to - it's worth it)
Casper, WY (stop by some lunch or an oil change - holler first!)
Medicine Bow, WY (Home of the "Virginian Motel")
Saratoga, WY (hit the hot springs).
Then South to the Back roads of Colorado to C-springs.
You'll likely find small towns along the way at most every highway intersection. Each will have it's own color, but likely very light on 'attractions'. You'll do well to find gas 'n snacks outside of the 'bigger' places I've listed.
Way the Interstates go - you won't save much appreciable time by running down I-25 from Casper to Cheyenne and across to Saratoga. But, if you've a thin for 'super-slabs' - it works.
I was born and raised in Laramie, my folks still live there. Highway 287 to 14 is a road I'm reluctant to take in a cage. We call that piece of road the 'Meat Grinder' due to all the traffic accident and fatalities there. Google up the stories of the UW track team, professors and others that've died there. Big effort going on to fix it up somehow. The road from Laramie to Saratoga over the Snowy Range - wonderful. Stop in at Centenial and have a GREAT steak dinner at The Old Corral Hotel & Steakhouse.
Check this site for free PDF highway maps