I made the mistake of coming into St. Mary on 89 from the South. Do NOT do that.
Course it didn't help that I was pulling the swivel wheel and there were bicyclists on the switchbacks. What a nightmare.
464 is nice and easy.
As far as out of Yellowstone, 191 is a beautiful road, but busy and tricky for big rigs. 287 is much more flat and straight. Different type of beautiful, but would be much easier with a big rig. Rode both on the Goldwing while out there.
A cool/disturbing thing to see is on 287. Earthquake lake. You can still see where the mountain fell into the river.
Earthquake lake