Gaga is more iconic in that she has generated and contributed more iconography through videos, outfits, performances, lyrics, stunts, message and general persona.
I'd say Rihanna is more Beyonce in that the few iconic things they've done are huge, and they're loved...but the likes of Gaga, Britney, Madonna and MJ just exude iconography because they're such visual artists with such attention surrounding their personas.
ETA: Not people measuring iconography by success.
