What is the Busiest National Park in England?

There are 15 national parks in the United Kingdom, 10 of which are in England. The following parks are in England:

Northumberland

Lake District

Yorkshire Dales

North York Moors

Peak District

Broads

South Downs

New Forest

Exmoor

Dartmoor

This information can be found on the National Parks website for the United Kingdom. In addition to national parks, the pdf link provided also has "Areas of Outstanding Natural Beauty" marked.