Hard to say. Grand Canyon is pretty impressive, but I have a fondness for the Washington Monument

So many, it is hard to isolate. Yosemite, The Arches, Grand Tetons, Grand Canyon, Crater Lake and Yellowstone.  I could name more but those come to mind first.

Grand Canyon for natural beauty.  Gettysburg and Antietam for the history.  Bill

Lincoln Memorial

Chaco Canyon in New Mexico

I live next door to Mt Rubidoux and a natural Wildlife Corridor that just does it for me daily both for hiking and biking....

Too many - can't choose. Last one I visited was Antietam National Battlefield.

Josef Katz The sites in Washington D.C.

Of course Grand Canyon is practically in my backyard and something all should see. I personally like Hearst Castle/Morro Bay a lot.

Acadia National Park

Katahdin Woods and Waters National Monument

There are so many. As a child I loved Lassen in Northern California.

I really do like Mount Rushmore however I've only been there once and I teenager...

I love Central Park in San Mateo; also I like Pacifica Pier. There is other another one I love too.

San Mateo Central Park

Japanese Tea Garden San Mateo, CA

Pacifica Pier

I've been to many of these, from the National Archives, to Gettysburg to the Grand Canyon (aka "The Big Hole in the Ground").  So many to choose from, each with their own uniqueness.  I can't pick one.

More than just one, but the Grand Canyon might be my favorite.

I love visiting Yellowstone and get there about once every 3 years, but i get to visit the White Mountain National Forest here in NH year round

Too many to choose from.

Gettysburg

the Washington Monument and many others in DC....  I've seen the Grand Canyon too.... ...

The Statue of Liberty. But all the DC monumnets are gorgeous, Lincoln, Jefferson, Washington.

So far Yellowstone

really? I saw that as a kid & was unimpressed. 

I'll give my vote to the hoo doos in Utah.

No favorites but we do have a big selection in DC.

Yosemite, Grand Teton, Yellowstone and I live in the Cleveland National Park. I couldn't pick only one. Josef Katz  Are you a hiker? 

For me, it would be the Grand Canyon National Park Josef Katz. 

I don't have one.

None.

I walked under the Gateway Arch when I was hitchhiking across the country at age 17!

Favorites?  Wow, that's a toughie.  Places I have been would include anything in Arizona and Utah, including the Grand, the Arches, Bryce, Zion (I waded up The Narrows).   California, including Yosemite (I went swimming in the Emerald Pool!).  Cades Cove in Tennessee (jump off the cliff into the pool at the bottom of Abrams Falls.  And of course I live in Florida, where we have The Everglades, Pennekamp, St. Marks, and so many more.  Did I mention that I went to the Rio Grande in New Mexico?  And Big Bend Texas?  Oh, so many places!

I've been the most moved at Arlington and the Veitman memorial, but for beauty I'd say grand canyon, but I also loved Arches and Devil's Tower for their unique natural formations and Mamoth Cave was pretty cool as well.  There are still several I have not been to yet.

The Grand Canyon

Grand Tetons, Grand Canyon (north), Glacier National Park

hmm Grand Canyon, Pikes Peak, shenandoah park, Skyline Drive, Jefferson Memorial.. to name a few

Gosh, there are so many, Bryce & Zion, Rocky Mountain Nat'l Park, and of course Grand Canyon.

Yellowstone National Park.

Many.

Yosemite is incredible.  There are so many great national parks but Yosemite is my favorite so far.  I want to go to Yellowstone soon. 

Yellowstone National Park is my favorite...and right next door!

The Everglades!

Fairbank House in Dedham MA....Oldest stnding stucture residential ....ceiling height about 6 ft...ish.....

There are many, it's hard to pick a favorite.

Have not seen enough to have an informed opinion. 

Many

I like the Smokey Mountain National Park.

Hard to say as I have only been to a few.

Since i'm in CA it will be : 

California Coastal

and

Giant Sequoia.