Best KOA I've been to. Period
Without going into the plethora of reasons that you should visit Yellowstone, let me say that this campground stands on its own as a family-approved space. The views are fantastic (can you go wrong out in that part of the country) and the amenities are unparalleled in a campground.
There is every type of facility that you can imagine: washing, hot showers, dining, store, indoor heated pool, hot tub, etc. I mean, is this even camping? I was less than impressed with those features when we arrived and more with the fact that I was with my teenage daughter, younger brother, and father to see Yellowstone NP, but boy did that indoor pool and come in handy when it rained and got a bit chilly. The hot tub helped ease the hiking woes my body was taking in from the long hikes in the area.
Did you know there are so many stars? You feel like you can see them all from that darn KOA. The energy of the campground is palpable as the sites are only separated by a rustic wood fence. It is a busy place, so I was okay with that part. These days I prefer to be away from everyone or at least further 10', but my daughter loved it.
The main thing here is that I would recommend staying within Yellowstone if that is your purpose of your visit. It does book up fast, though, so this is still a viable option. I think we paid more for the KOA site vs. Grant Campground at the southern end of Yellowstone a couple of years later, but there is no comparison to what people paid to stay in town near the West Gate to YNP.
Oh, be ready for a LOOOONNGG wait at the YNP gate and DEFINITELY have your annual pass already purchased. We had a pass so we were able to skip about 30 minutes of a waiting to get in, but you still need to navigate four roads converging 1-2 miles out from the park's entrance. You've been warned.