If you can stand no sewer, limited water (spigots around), and electricity only by generator during the day, Mammoth is fabulous. We stayed there one year, but our favorite place to stay is at Yellowstone River CG just north of Gardiner. It's probably the closest. I thought Fishing Bridge was very cramped as the only full service park in Yellowstone, but some like West Yellowstone.
To be honest, it doesn't matter. YNP is the granddaddy of all parks for a reason - it's just a lot of fun and very beautiful!