Yosemite - not only because its in my back yard and I grew up there. It's the only NP that I have been too that you are immersed in the natural beauty with-out a lot of travel.
Wherever I stand I have the best view.
2001 25dbs Tahoe TT, 2005 Chevrolet Silverado 2500 Duramax
Zion first, then the rest of the Utah national parks. Yosemite, Glacier. My only problem with the Canadian parks is how far I have to travel from SW Oklahoma. There are so many state parks in between, that I would have a hard time picking even a double handful.
2010 F250 6.4l Diesel
2012 Keystone Alpine 3600
Ray, Kay, and Tessa & Reba. Tessa is a 5 year old American Cocker. She thinks that she is the co-driver. Reba is an 9mo old Australian Shepard that is learning about traveling.
All the ones mentioned are great but almost all are full of people - the ones that we prefer are like Great Basin that is just as impressive and far less crouded.
Bob/Beavercreek OH
2011 Silverado 1500 LTZ/Trail Lite 8230
#1, Arches. Yosemite, Tetons, and Glacier are also favorites. But there are so many I haven't even been to yet... like Isle Royale, for example, and that's one I was thinking about for this summer.
Mike & Sherry
2000 Mercury Mountaineer
2008 Toyota Highlander
2011 KZ Spree Escape E14RB