Tony Stark, by definition, has to be the biggest playa. He feels absolutely nothing for the chicks he bangs, he just has an itch to scratch or some devious scheme in mind where they might be useful. When he doesn't need them anymore, he discards them like used dishrags.
All the others actually have feelings for the chicks they date, but Tony doesn't love anybody except himself - therefore, like James Bond, he is the biggest playa.