At first Britney Spears, then Christina Aguilera and Madonna, and at the third place Mariah Carey.
What the hell is Beyoncé and Lady Gaga doing here?
Beyoncé already had a career with Destiny's Child, it's ridiculous to put her here. If you're going to count her, then you must count Gwen Stefani or Fergie, whose debut solo singles are most iconic by miles. You can count Shakira's first English single (Whenever, Wherever) which is more iconic too.
And Lady Gaga, if "Poker Face" would had been her debut single, ok, but "Just Dance" (being one of her best songs) isn't iconic, for her career, for pop music or for anything. "I Kissed A Girl" by Katy Perry is most iconic than both of them together.