State For All The Natural Environment

Rhode Island (RI) is the best state in term of natural environment -- low pollution, high quality air and water.  If you're not planning to move there but only to visit areas with protected natural wonders, then there are 62 National Parks across the country, most of them are UNESCO World Heritage and/or UNESCO Biosphere Reserves. 

These are some of the parks we've visited and liked:

- Arches;  Bryce Canyon;  Zion (UT) 
- Biscayne;  Everglades (FL)
- Carlsbad Caverns;  White Sands (NM) 
- Channel Islands;  Death Valley;  Joshua Tree;  Kings Canyon;  Redwood;  Sequoia;  Yosemite (CA) 
- Crater Lake (OR)
- Glacier (MT) 
- Grand Canyon;  Petrified Forest;  Saguaro (AZ) 
- Grand Teton;  Yellowstone (WY) 
- Great Smoky Mountains (TN) 
- Haleakala,  Kilauea,  Mauna Loa volcanoes (HI)
- Mesa Verde;  Rocky Mountain (CO) 
- Mount Rainier;  N. Cascades;  Olympic (WA)

There are also 150+ National Forests, too long a list to write here.

BTW, even though RI is ranked the best state in term of natural environment, it's one of 22 states without a National Park.  The absence of a NP doesn't seem to affect the state's solid image at all.

However, IMO, CO is better than RI if one wishes to surround oneself with natural wonders. 

There are 58 fourteeners (14ers) in the state (14ers are mountain peaks that exceed 14,000 ft or 4,000 meters in height), all of them are hikable.  It has four strikingly beautiful NP, seven major rivers, and a dozen waterfalls.  It has mountains, plains, forests, and deserts.  Some parts of the state still have the look of rustic Wild West towns while other parts are as pretty as a Norman Rockwell painting, and then there's Denver, a true cosmopolitan city. 

CO has decent public transportation, 4 clearly defined seasons, clean air and clean water.  Best of all, breathtaking sceneries are practically in one's backyard and everywhere one looks.