That's a tough one for me. I'm a big Route 66 fan, but I also love Yellowstone and the Grand Tetons. You could even go a bit out of the way and make a west coast Pacific Coast Highway thing out of that. I have a couple of links that might help you decide:
A Journey Along Route 66 in Pictures is a great posting of almost all attractions and countryside you'll find along the Mother Road from end to end. Here's my own itinerary for Colorado and Yellowstone:
Colorado, Wyoming, and South Dakota - September 12 - 21 2007 and related
photo gallery which includes Yellowstone and the Tetons, among others. Here's a fellow that actually combined both Route 66 and Yellowstone into one trip: [url=http://www.