Grizzly RV park is an excellent RV park. It's location is just two blocks from the West Entrance to Yellowstone.
Here's a photo I took at Grizzly RV park last August.
I don't have a favorite place to visit other than the entire park. When you visit Old Faithful Geyser be sure to go into the lobby of the Old Faithful Inn, it's really impressive. As a matter of fact, one of my favorite spots for viewing and eruption of the geyser is on the second floor veranda of the Inn with an adult beverage in my hand. You don't have to be a guest at the Inn to use the second floor veranda.
Here's a pic of Old Faithful erupting that I took from the veranda.
I also love to visit the Lower Falls on the Yellowstone River. You can view these falls from a number of different vantage points, including a short, steep trail that ends right at the top of the falls.
Here's a pic I took last year of the Lower Falls.
Here's a photo of the viewing area at the brink of the Lower Falls.