I found interesting list of 10 biggest airports in the world by the ammount of passengers in 2006 and by the ammount of international passengers. Notice, that if you count the domestic air traffic, biggest airports are in the United States, with 5 airports in the list. If you take a look at the international traffic, neither one of top american airports are qualified.

So here is the list by the ammount of passengers:

1. Atlanta – Hartsfield – Jackson International Airport (USA) – 84 846 639
2. Chicago – O’Hare International Airport (USA) – 77 028 134
3. London – Heathrow (England) – 67 530 197
4. Tokyo – International Airport (Japan) – 65 810 672
5. Los Angeles – International Airport (USA) – 61 041 066
6. Dallas – Fort Worth International Airport (USA) – 60 226 138
7. Paris – Charles de Gaulle Airport (France) – 56 849 567
8. Frankfurt – International Airport (Germany) – 52 810 683
9. Beijing – Capital International Airport (China) – 48 654 770
10. Denver – International Airport (USA) – 47 325 016

And here is the list by internationa passengers:

1. London – Heathrow (England) – 61 348 340
2. Paris – Charles de Gaulle Airport (France) – 51 888 936
3. Amsterdam – Schiphol Airport (Holland) – 45 940 939
4. Frankfurt – International Airport (Germany) – 45 697 176
5. Hong Kong – International Airport (China) – 43 274 765
6. Tokyo – Narita (Japan) – 33 860 094
7. Changi Airport (Singapore) – 33 368 099
8. London – Gatwick Airport (England) 30 016 837
9. Bangkok – Suvarnabhumi Airport (Thailand) – 29 587 773
10. Dubai International Airport (United Arab Emirates) 27 925 522

Notice that two airport from this second list are London airports… This year air traffic probably is not so big because of global recession… Price of tickets jumped for most destinations. People travel less. Probably situation will not be much better until next year.
Cheap airline tickets in these times are not easy to get.

I got this airport lists from google answers.