I would say Toxic, but it is a masterpiece, so I don't think it's really overrated. Same with Womanizer, it made her career comeback. So I think Slave is her most overrated one.
Please...Womanizer is one of her best...definitely top 5
Toxic and Circus are SEVERELY overrated...especially the latter. I never got into Circus...and why people praise it, I have no idea. As for Toxic...people swear it's her best single, when that belongs to 3. And that's not even her best song, Get Back is by far the best of her full catalog