Ennis-W. Yellowstone (287) ~70 mi. map A great drive once you leave the tourist-choked Yellowstone area... a quiet road that goes down the middle of a very scenic valley with the mountains rising steeply nearby. The road partially follows a fast-flowing stream.
Montana offers many other scenic drives, especially on its western half but I regret the lifting of the no-speed-limit allowance it had for a couple of years. Apparently too many drunken teens with their Camaros and Ford Pickups ruined it for the rest of us.