City population figures are pretty useless for most purposes as a city is merely a political unit. (The City of London, for example, has a population of around 8,000... Yes, eight thousand.)
In the US, the figure that is most useful is that of the primary census statistical area. Here are the top 25:
1. New York-Newark-Bridgeport, NY-NJ-CT-PA - 21,961,994
2. Los Angeles-Long Beach-Riverside, CA - 17,755,322
3. Chicago-Naperville-Michigan City, IL-IN-WI - 9,745,165
4. Washington-Baltimore-Northern Virginia, DC-MD-VA-WV - 8,241,912
5. Boston-Worcester-Manchester, MA-RI-NH - 7,476,689
6. San Jose-San Francisco-Oakland, CA - 7,264,887
7. Dallas-Fort Worth, TX - 6,498,410
8. Philadelphia-Camden-Vineland, PA-NJ-DE-MD - 6,385,461
9. Houston-Baytown-Huntsville, TX - 5,729,027
10. Atlanta-Sandy Springs-Gainesville, GA-AL - 5,626,400
11. Miami-Fort Lauderdale-Pompano Beach, FL - 5,413,212
12. Detroit-Warren-Flint, MI - 5,405,918
13. Phoenix-Mesa-Scottsdale, AZ - 4,179,427
14. Seattle-Tacoma-Olympia, WA - 4,038,741
15. Minneapolis-St. Paul-St. Cloud, MN-WI - 3,538,781
16. Denver-Aurora-Boulder, CO - 2,998,878
17. San Diego-Carlsbad-San Marcos, CA - 2,974,859
18. Cleveland-Akron-Elyria, OH - 2,896,968
19. St. Louis-St. Charles-Farmington, MO-IL - 2,866,517
20. Tampa-St. Petersburg-Clearwater, FL - 2,723,949
21. Orlando-Deltona-Daytona Beach, FL - 2,693,552
22. Pittsburgh-New Castle, PA - 2,446,703
23. Sacramento-Arden-Arcade-Yuba City, CA-NV - 2,397,691
24. Charlotte-Gastonia-Salisbury, NC-SC CSA - 2,277,074
25. Cincinnati-Middletown-Wilmington, OH-KY-IN - 2,176,749
The full list of 719 primary census statistical areas is here.