[1) Mammoth - Kentucky, 2) Jewel Cave - South Dakota, 4) Wind Cave - South Dakota] and three smaller caverns. In the past, before getting married, I visited Carlsbad Cavern in New Mexico.
The cavern that has the most beautiful and colorful formations is Blanchard Springs Cavern outside of Mountian View, Arkansas and a great "plus" to the deal is that the roads to and from are full of twisties and elevation changes!
Life magazine once stated that Blanchard Springs Cavern was "one of the most extraordinary finds of the century" and I would tend to agree. The cave has two paved routes. Both routes take you into a living cave with slow changing stalactites, stalagmites, columns, and flowstones. The cavern stays at a constant 58 degress Farenheit with 97-100% humitity. The size of the cavern is not what it is known for, but the beauty of the formations is.
Now the ride to get there is awesome. We took AR 16/9 from Clinton, AR to Mt. View and greatly enjoyed the 35 miles of scenic windy roads. When we left the caverns we were riding back to Mena, AR. We rode west on AR 14 to Harrier then west on AR 27 to Marshall. At Marshall we took AR 74/16 west to AR 7. AR 16 is awesome - see picture below. We head south on AR 7 to Russellville and on to Mena.
If you get the chance to ride up to Mt. View you will greatly enjoy the ride, but make time to visit one of the most beautiful caverns in America - Blanchard Springs Caverns.