Attendance isn't what determines the flagship title, but the amount of things to do. That's why WDW is the flagship resort. It's got four parks, at least 20 resorts, golf courses, water parks, etc. It's the flagship resort by a mile. Disneyland has the most things to do in one park, more than any of the other Disney parks, making it the flagship park.
The best or most important thing owned or produced by a particular organization.
I do not find this descriptive of Disney World.