My personal opinions are Merlene Ottey and Gwen Torrence for sheer consistency. Merlene Ottey ran sub22 seconds 16 times, more than any other woman in history, and Gwen Torrence ran sub22 10 times.
In terms of gold medals, Allyson Felix and Veronica Campbell-Brown are more impressive, but Allyson has run under 22 seconds just 6 times and Campbell-Brown only 4 times.