Yellowstone was our first park established in 1872.  It has Old Faithful and a majority of the world's geysers.  It is home to about 300 grizzly bears, black bears, wolves, pronghorns, mountain goats, mountain lions, coyotes, bighorn sheep, trumpeter swans and herds of elk and bison.  In summer make sure you stay a safe distance from the bears, especially if the sow has cubs with her.  In winter you can rent snowmobiles in West Yellowstone and be guided into the park.  I have used Ken at Yellowstone Adventures several times.  The bubbling mud pots are fun to watch.  Make sure you bring binoculars with you so you can view wolves and bears at a safe distance.  My family has had two reunions there, the first when I was around 8 years old.  That was when the bears would come to the dump and scavenge for food.  People actually used to get their photos feeding the bears. I have been there nine times in winter and three times in summer.  No matter when you go you will have a great time