It depends upon what you want to do. There are older CG in Yellowstone and this causes a problems with a lot of the larger rigs out there. Most of the park CG are small and crowded. We were just up there this spring and were shocked at how small the sites are at FB.
I would recommend you figure out what you want to do then stay at several good spots from there. West Yellowstone has some great spots, Grizzly is one, and we love the northern tier (we love to photo the animals all along the northern tier and hardly go to the Geo-thermal area at all), and so we stay at Gardiner at
Yellowstone River RV Park. This park has a lot of park employees staying there so you will need to make your reservations early (they open April 1 for reservations and the park opens May 1st, I think), call them April 1st and make your reservations as soon as you can. You are 5 minutes from the north entrance and after years of visiting YNP, this is our "home" there.