National Park Statistics
In case you were wondering, the 2002 visitation statistics for U.S. National Parks are now available.

The Top 10 Across All Parks (Park, Historic Site, Monument, etc...) are:
1. Blue Ridge Parkway
2. Golden Gate National Recreation Area
3. Gream Smoky Mountains National Park
4. Gateway NRA
5. Lake Mead NRA
6. George Washington Memorial Parkway
7. Natchez Trace Parkway
8. Delaware Water Gap NRA
9. Gulf Islands National Seashore
10. Cape Code NS

Of the 10 on that list, I have been to 4, although not necessarily in 2002.

Strictly by National Park, the Top 10 were:
1. Great Smoky Mountains
2. Grand Canyon
3. Olympic
4. Yosemite
5. Cuyahoga Valley
6. Rocky Mountain
7. Yellowstone
8. Grand Teton
9. Zion
10. Acadia

I've been to 3 of these, although when I visited Cuyahoga Valley, it was still designated as a National Recreation Area.