@Yoda: I googled 'most popular dances' or something like that and some resulting articles suggest that tango and jazz have some degrees of improvisation...
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.