@Yoda: I googled 'most popular dances' or something like that and some resulting articles suggest that tango and jazz have some degrees of improvisation...
Yeah, it's tricky because in some ways any time they allow for improvisation, they allow for the person to dance in a style outside their own, too, I suppose. But that's interesting.

Interpretive Dance comes to mind, too. I don't think that has to be improvised but I'll bet it's more improvised than other forms.

BTW, since you're here, I'd like to ask whether there is any way to search, or filter, members' favorite movies? Other than having to go into each person's profile page manually.
Yep: just go to Movies (found in the header/footer), search for a film, and everyone with it favorited will show up on the resulting page (bottom right).