These are what I could think of to add:
10) William Thurston
11) John Milnor
12) Walter Rudin (Wrote the gold standard Analysis texts)
13) Grigori Perelman for sure!!
14) Richard Hamilton
15) Marston Morse (Died 1977)
16) Bertrand Russel (Died 1970)
17) Michael Spivak (another amazing author- hands down one of my favorites)
18) P. R. Halmos (Died 2006)
19) Ian Stewart (Publishes many mainstream books as well as texts)
20) Steven Krantz (A prolific author of both mainstream and textbooks)
21) Jerrod Marsden
1.Roger Penrose
2.Edward Witten (well, theoretical physicist)
3.Stephen Hawking
4.Paul Cohen
5.Alain Connes
6.Brian Greene
7.George Lakoff (not exactly a mathematician by formal training, but important)
8.Lee Smolin (another theoretical physicist)
9.Andrew Wiles
Look them up on Wikipedia if you haven't heard of them.