Yellowstone NP, WY *****
Pros: this is one of the best park in the US and possibly in the world, which will surprise and astound you; fantastic wildlife; limitless photo taking opportunities
Cons: weather can ruin your plans, be careful with the local wildlife, some popular places e.g. Old Faithful can become too crowded
Taken from the official website… ” 1872, Yellowstone became the first national park for all to enjoy the unique hydro-thermal wonders. This is such a dynamic place that every visit to Yellowstone brings new sights, sounds, and memories”. It’s visited by ~4 million (!) visitors every year (2018).