I was poking around the Internet and discovered the government's site where statistics can be found on national parks.  Just for grins I went through the list of parks and pulled out all parks that had greater than 3 million "recreation visits" in a year (the data is from 2005). 

BTW, if you're curious about the designations.  Here's the key:

One of the things that jumped out at me was what is considered a national park. I believe all the most highly visited parks are actually highways. I assume that's the case for Golden Gate NRA.  I know it's the case for the parkways (PKWY). I'm curious whether the states get more funding based on these statistics or whether it is the other way around.  They generate more funding because they charge people who drive on these highways.

If you pull out the parkways, the list is not all that intuitive.  For example, I knew Lake Mead was popular but who would guess it would rank higher than the memorials in DC?  There must not be an accurate way to determine the true number of visits.  Come to think of it, last time I was in DC I don't remember paying to see the WWII memorial.