Yellowstone National Park is one of those few places which is a "must see before you die". I have been to this place couple of times and willing to go again. These shots were taken during my Labor day weekend 2014 trip when it was mostly cloudy. I am not sure about the winter conditions here but surely can recommend visiting any time during the year when its not too cold. This park has everything - lake, multiple huge waterfalls, wildlife, natural beauty, mountain views and above all geological activities which you can see only here. 

I have visited YNP from Denver which is a 10-11 hour drive with stops. Both the times when I drove there, I drove in a single day as I wanted to reach there asap. I have added the map of route I have taken at the end of the post and I recommend it because this trip is scenic and one cannot get bored during the long drive.

Scenic Route from Denver to Yellowstone National Park