The thing would be what dance should you do?
It might be somewhat corny but I haven’t seen a flash mob do the “electric slide” or “The Bartman” (ok…. that last one is the corniest of the two).
Lately people have done Michael Jackson, GaGa, even madonna and such. Try something old and funny. Just stay away from “The Macarena”.
It might depend on where you are and what is and popular there. Whatever is most popular will get you the largest response and get more participants familiar with what you want to do.