Most overhyped: Old Faithful, Yellowstone National Park
Don't get me wrong, Yellowstone is a fantastic park. I could spend a month there and not get bored. Lamar Valley, Grand Canyon of the Yellowstone, Grand Prismatic, Mammoth Hot Springs, etc. are otherworldly. But Old Faithful is supposed to be the star attraction and really, it's just a 2 minute show of hot water blasting out of the ground in a really plain part of the park and then nothing. It draws huge crowds though because of its reputation. I've been there twice and both times the crowd has given it a collective "meh" when it ended. I get that it's iconic and that you probably have to do it once just to check that box, but's it's incredibly underwhelming.
Most underhyped: Little Bighorn Battlefield National Monument, Crow Agency, Montana
Made a late route change this past summer when driving through the US to stop by the Little Bighorn Battlefield in the Crow Agency, about an hour east of Billings, MT. As a fan of stories of the old West, I was interested in seeing the place where so many iconic figures came together. General Custer, Sitting Bull, Crazy Horse, Chief Gall, Lame White Man, etc. all in the same place at the same time.
We weren't expecting much but had a terrific experience. It wasn't crowded, maybe 100 people, which made for a casual, stress-free visit. One of the park rangers was a descendant of one of the natives that fought against Custer. He spent about 30 minutes walking us through the lead up to the battle, the initial engagement, and finally Custer's arrogance and errors in judgement that led to the massacre then left us to wander the battlefield at our own pace. I had no idea the battle site was so massive, but the park has done an amazing job of constructing a road through the battlefield that respects the places where US army and natives were killed and allows visitors to see the whole battlefield in one visit. Along the road you can stop, get out of the car and read a description of the significance of each location in the battle.
In each spot where someone was killed they have placed a headstone to commemorate the death. I was struck by just how many headstones there were, and how scattered they were over so many miles. You could stop anywhere along the road and spot a solitary headstone off in the distance no where near the main battles and wonder about the circumstances of their death. The hill on which Custer's last stand took place was really the only spot where there was a large collection of headstones, with Custer's the only one in black (photo below). It was incredibly sobering to see it first hand. I noticed several visitors overcome with emotion at the scene.
Anyway, maybe it's just me but while the battle has certainly been studied to death in so many books, I had never seen the actual site promoted as a tourist attraction before and was completely surprised at how respectful it is of both sides of the conflict. A terrific, understated gem. If you're venturing out to Montana some day I'd highly recommend you spend a few hours here. It's well worth it.