National Parks can only be created in a bill passed by Congress. Any president, however, may declare federal land parcels to be National Monuments. Most of the recent new national park designations have been a result of legislation with bipartisan sponsors from an individual state, with the main (but unstated) reason being to promote tourism.
Several National Parks are only worthy of National Monument designation IMO, and I believe a few (e.g. Gateway Arch) should be in a different category entirely.
I have been to 49 NPs, quite a few of them more than once. I was at a few of them while they were still national monuments. Probably among the lesser-known parks I've been to include Wrangell-St. Elias, Great Basin, Theodore Roosevelt, and Congaree.
Some of the parks are universally loved such as Yellowstone, Yosemite, Glacier, Grand Canyon, Crater Lake, & Bryce Canyon. A couple which I think impress me more than most people include Canyonlands & Redwood, both of which I've visited multiple times.
Beauty, significance, or uniqueness are sometimes in the eye of the beholder.
I really liked Congaree, for example. I'm especially interested in plant ecosystems, and there's no significant lowland bottomland like it remaining elsewhere in the southeast. I had never seen a forest similar to it before. There are also some impressive old growth trees amidst the lush vegetation, though they're scattered and not concentrated. Another park which impressed me much more than I expected was Badlands.
Two adjacent state parks, Na Pali Coast and Waimea Canyon, on Kauai, are very much National Park-worthy IMO.