There's a reason Lifey suggested checking out the National Parks website, so you can explore for yourself and see which ones you think are most interesting. The fact is that any place that has been selected as a National Park has earned that distinction because they are unique attractions that you won't find anywhere else.

I will say that based on your current route, I'd start by giving some consideration to the parks of the 4 corners area. For example, after Santa Fe/Albuquerque, you could go up to Great Sand Dunes and/or Mesa Verde in Colorado, and then go through Monument Valley (Tribal Park, not National Park, as it is on Indian Lands) on your way to the Grand Canyon. Or if you stick to I-40, the Painted Desert and Petrified Forest are right along the way.

Between Grand Canyon and Las Vegas, you could head up into Utah and check out Zion or Bryce Canyon.

Those are just a few of the options, that are close to the path you are already looking at, but I would urge you to follow the previous suggestion of spending a little time researching the parks yourself to see what looks interesting to you.