ATL and ORD are pretty close to a tie for #1, and they have switched places a few times in the past if I remember correctly.

I was in ATL a few weeks ago and this lady asked me if Los Angeles (presumably LAX) was the busiest airport. I replied that ATL is the busiest, and Chicago (ORD) is second. She said that she thought Chicago was the busiest and her boyfriend thought Los Angeles was the busiest.