Sueshore, it was breathtaking. I've never been there before, but I imagine the winter is the time to see it. As I mentioned in that other thread the contrasts of hot and cold are so unbelievable. Pictures don't do it justice - you can't capture cold, and I mean really cold, on film.
The Gros Ventres valley outside of Jackson Hole was even better in a way. Much more wildlife and a different type of scenery, plus snowmobiling. We didn't get to snowmobile Yellowstone because it was so cold.