Oh, mostly the forest-y and nature-y states - Arkansas, Colorado, New Mexico, Utah, Idaho, Tennessee, Kentucky, Arizona, Washington, Oregon, California...I would've liked to see more of Montana and Michigan, but it gets cold up there in the winters.
Mostly I drove through Texas to avoid Oklahoma (one of my least favorite states), but the Guadalupe Mountains were very nice.
I'm not sure I'd call it 'clean' though; the air seemed awfully smoggy and I saw almost as many flaring wells as I did in North Dakota. But there were lots of open spaces and the people were friendly.
Washington, Arkansas, New Mexico, and Utah get my vote for the best places to do a parks/forests vacation, depending on your interests.
The air was “smoggy” driving through Texas? Uh huh... I’m going to go ahead and put that on the same level as your claim about Texas being lax on litter penalties.
By the way, Texas has more acres of forest land than every state you mentioned as being nature-y and forest-y and having more incentive to care about the environment. The only state with more forest than Texas is Alaska.
Like someone else said, Texas is too big to make generalizations.
Texas has amazing (and clean!) state and national parks as well. Definitely worth a visit, especially Big Bend.