I just found interesting list of largest and biggest cities on the world. Here are first twenty sorted by the estimated population at 01.01.2009. These are approximate figures so data on other sites can be different according to the different methods of measuring.

1. Tokyo (Japan) 33 800 000

800px tokyo aerial night 300x199 20 largest and biggest cities on the world in 2009
2. Seoul (South Korea) 23 900 000

800px gangnam1 300x195 20 largest and biggest cities on the world in 2009
3. Mexico City (Mexico) 22 900 000

800px santa feconj 300x200 20 largest and biggest cities on the world in 2009
4. Delhi (India) 22 400 000

800px raj path 300x225 20 largest and biggest cities on the world in 2009
5. Bombay (India) 22 300 000

800px mumbai downtown 300x208 20 largest and biggest cities on the world in 2009
6. New York (USA) 21 900 000

800px lower central park shot 5 300x225 20 largest and biggest cities on the world in 2009
7.  Sao Paulo (Brazil) 21 000 000

800px sao paulo business district 300x190 20 largest and biggest cities on the world in 2009
8. Manila (Philippines) 19 200 000

manila 300x199 20 largest and biggest cities on the world in 2009
9. Los Angeles (USA) 18 000 000

los angeles 300x225 20 largest and biggest cities on the world in 2009
10. Shanghai (China) 17 900 000

800px shanghai nanjing road 300x220 20 largest and biggest cities on the world in 2009
11. Osaka (Japan) 16 700 000

800px peopleinosaka 300x199 20 largest and biggest cities on the world in 2009
12. Calcutta (India) 16 000 000

strandrdskylinegk7 300x208 20 largest and biggest cities on the world in 2009
13. Karachi (Pakistan) 15 700 000

karachifinancial 300x225 20 largest and biggest cities on the world in 2009
14. Canton (China)  15 300 000

800px guangzhou dusk 11 5 2008 300x147 20 largest and biggest cities on the world in 2009
15. Jakarta (Indonesia) 15 100 000

800px jakarta25 300x165 20 largest and biggest cities on the world in 2009
16. Cairo (Egypt) 14 800 000

774px view from cairo tower 31march2007 300x232 20 largest and biggest cities on the world in 2009
17. Buenos Aires (Argentina) 13 800 000

palermos neighbourhood skyline 300x225 20 largest and biggest cities on the world in 2009
18. Moscow (Russia) 13 500 000

moscow kremlin 5 300x225 20 largest and biggest cities on the world in 2009
19. Beijing (China) 13 200 000

beijing 300x189 20 largest and biggest cities on the world in 2009
20. Dhaka (Bangladesh) 13 100 000

dhaka skyline aymash 300x225 20 largest and biggest cities on the world in 2009
Complete list of largest cities in the world you can see at

http://www.citypopulation.de/world/Agglomerations.html