Stanning aside, I think the answer is Lady Gaga. Madonna in her earlier years was very influential, but I don't see her having the same influence over pop culture and the world in general like she used to.
Oh I didn't even see Madonna as an option. It's clear she's had the most influence (so far) but in this day and age, I don't think she has as much as the current batch of pop girls. This isn't a bad thing, at all.