I've been to around 2/3rds of the different National Parks, but haven't been to any in Alaska, so i'm sure some of those would be here. I think i'd also have Dry Tortugas on this list, when I finally get around to seeing the coral reefs there with my own two eyes.

But, here's my list.

Glacier - by far and away the best place i've ever been to.  The scenery is suburb, it contians all of it's native wildlife before the Euroman encroachment epoch, is not overdeveloped to death, and scrambling up those peaks is AN experience.  It doesn't matter which peak, any of them - and you will see the park in it's grandeur by looking over it. I've been to it many times, and I never once have been bored with any of my experiences there.

Olympic - I was struck by this place, because it's got spectacular mountains, spectacular rainforests with large old growth trees, and a wild coastline that is very wild and primitive.  I lucked out one year by catching this place during the peak of the wildflower bloom on hurricane ridge, and it stood out to me as one of the most beautiful scenes on Earth.  So much to explore, you could spend a long time doing it.

Capitol Reef - By far the best park in the southwest in my opinion.  Especially the remoter backcountry - such as cathedral valley, and the remote slot canyons found in the southern part (crowds? What crowds?)  It's truly a gem of geological wonders.  Get off the main road, and you'll have large stretches of this place, where it's just you, the yote's and the lizards. This place is not for the "i need a paved road" crowd.

Yellowstone/Grand Tetons/Beartooth Wilderness/Absoraka Wilderness/Jeb Smith Wilderness - It's the eptiome of the Rocky Mountain experience, especially the backcountry when you leave the roads behind.  I can't just include Yellowstone here, because I also found many similar experiences within the Greater Yellowstone, and don't exactly need to be within the park boundary to find grand experiences.  But the Greater Yellowstone is an experience.  I truly miss having this place being just a few hours drive down the road. 

Sequoia - While the trees are impressive, it was the mountainous maze within the backcountry that captivated me.  In terms of mountainous National Parks, only Glacier would I rank higher.

Channel Islands - i've only been to Santa Cruz, and only spent 3 long days, and 4 nights on it, but during the time I spent here, I wanted to see a lot more of this place.  Very interesting place.  Just go during early spring or after the rains when everything is blooming and fresh.  The sea life surrounding these islands is also spectacular.  I'm sure scuba diving here would be mindbogglingly awesome.  We saw Whales, numerous seals, and thousands of dolphins, tons of starfish along the rocky shoreline not far from the kelp beds. I plan on going back here, and going to some of the other islands. 

Canyonlands - Utah's second best National Park. The White Rim trail is just a gem.  Everyone should attempt a portion of it once in their life.  Heck, the other parts of the park are amazing too. 

Great Smoky Mountains - Well, i've spent more time in this park than any other.  So, i'm biased, but the amount of life in the temperate rainforests is quite stunning.  While everyone seeks out black bears, etc in Cades Cove.  I'm not a huge fan of those areas, because of the crowds.  instead  I find the insects, amphibians, and flora of this place truly remarkable and more fascinating.  The AT through the park is stunning.  The heath balds are also grand, especially when the heath shrubs, like Rhododendron, Azaelas, and Mountain Laurel go into bloom in late may - mid june.  That's probably my favorite time of year, and its quite stunning, and exemplifies why this place is set aside as a biological gem.  Sure, the mountains are bigger in the west, but the flora and wildlife is in greater abundance here.

Congaree - the other park from the Eastern half that is a gem and not on many peoples radar.  Massive trees, and a lot of wildlife and bizarre species of birds, and insects that you just don't find in the dry interior west.  It's a turtle mecca too.  You need lots of slow moving water and muggy hot humid temps for these sort of creatures to thrive, and Congaree has loads of it .  Regardless, it's not a massive park, but it's definitely a unique place that protects a habitat that is getting increasingly more and more rare under the tune of the anthropocene epoch.  I plan on exploring more of this place over the years.

Great Basin - It's walking in those morraine fields with the ancient bristlecones that makes this place stand out.  You can find more interesting mountains elsewhere, but this place is very interesting.  It also has a remote feeling to it, compared to many other parks, plus who would guess there's a glacier in the higher reaches of Nevada.

Bryce, Petrified Forest, Crater Lake and Mt Rainier could also be switched into or out of the bottom 5, if I spent more time in those. 

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Parks i'm meaning to get to:  Gates of the Arctic, Dry Tortugas, Acadia, Lake Clark, Haleakala.

It's hard to see so many lists without North Cascades on them, and as well I'm one of the lucky ones who has been to NPSA [American Samoa] and it is a beautiful visit.

Have to limit it to national parks, or I'd never be able to get it down to 10. Hard enough even that way. In alphabetical order:

Acadia

Death Valley

Denali

Glacier

Grand Canyon

Great Smoky Mountains

Hawaii Volcanoes

Theodore Roosevelt

Yellowstone

Zion

My 1/50 of a dollar:

1.  Arches

2.  Great Basin

3.  Crater Lake

4.  Mt. Rainier

5.  Glacier

6.  Yellowstone

7.  Yosemite

8.  Grand Canyon

9.  Carlsbad Caverns

10.  Isle Royale

Assuming the crowds can be avoided:

Yosemite

Glacier

Yellowstone

Grand Canyon

Glacier Bay

Mt. Rainier

Olympic

Virgin Islands

Rocky Mountain

Grand Tetons

I've been to 28. I hope to add 12 more through this year.  So far, Glacier, Capitol Reef, and Badlands are high on my list. Death Valley, Yosemite, and the Yellowstone area would have to be included.  But I reserve judgement until I see them all...

GREAT SMOKY MTS,                                                                                                  YELLOWSTONE,                                                                                                        BIG BEND,                                                                                                                THEODORE ROOSEVELT,                                                                                      ZION,                                                                                                                          SHENANDOAH,                                                                                                          ARCHES,                                                                                                         CAPITOL REEF,                                                                                             CARLSBAD CAVERNS,                                                                                              OLYMPIC       

In no partucular order:

Yosemite

Olympic

Glacier

Grand Canyon

Zion

Yellowstone

Grand Teton

Sequoia

Acadia

Great Smokey Mountain

HM:  Capital Reef; Congaree, Everglades