RockOnTheNet.com has Madonna ranking at #1 as the top pop charting artist. The top ten are as follows:
#1. Madonna
#2. Janet Jackson
#3. Mariah Carey
#4. Whitney Houston
#5. Prince
#6. Michael Jackson
#7. Elton John
#8. Daryl Hall & John Oates
#9. Billy Joal
#10. Celine Dion
You can see the full list here.