So far my best has been going through the Colorado Rockies on the California Zephyr. That was some of the most amazing scenery I've ever seen between the mountains and seeing the Denver city lights just after exiting Moffat tunnel. I can't wait to do it again.
I haven't rode the trains outside of US, so Coast Starlight is the best one I've ever rode due to varities of landscape- ocean, Mediterrean style, bay in SF area, forest, volcanic peaks, etc. The best item in the train is Pacific Palour.
