I’m with shrapnel as I never tire of visiting Yellowstone.
I like Norris Campground inside the Park as it is at a convenient junction.
I like the last couple weeks of June with less crowds along with the bison and elk calves or September when the bulls start bugling. Mid July can be a zoo and mid August on not so bad...
The Bridge campground is convenient to the store and lake.
If coming through the East entrance a visit to the Buffalo Bill Center/museum in Cody is a must see.