Very OT, but it is National Park Week, so I thought it'd be fun to discuss the best of the US, especially as the weather is finally starting to warm in most of the country.
Where are your favorite spots? Any you haven't been impressed with?
I still have a lot to get to for the first time, but the best I've been to so far would probably have to be Zion in Southern Utah. Incredible hiking and such unique landscape. Angel's Landing is unnerving, but if you can handle heights, an all time favorite.
After that, I'd have to say Death Valley in California (Vegas is the closest airport) would be my second; it really pleasantly surprised me and is one of the more underrated parks. You can only really visit between November and March if you want to be active, but it's huge (largest in lower 48), extremely remote/quiet/peaceful, lots of unique landmarks (Dunes, Artists' Palette, mountains, Badwater Basin/salt flats) and even snowy hiking trails in the winter to juxtapoze with the barren desert floor (I visited in January, would definitely die there in July but some ppl think it's an experience to see the heat).
Although I don't think I've ever been to a national park/forest I didn't like, I think Joshua Tree in SoCal near Palm Springs is a bit lackluster. The trees are unique, but otherwise it's just normal desert and relative to the other parks in the region, doesn't pack much punch. Although it is a great park for people who like to camp, and you can do it most of the year (probably its biggest draw).