Funny, I hear "When I Think of You" all the time. I've always considered it a signature song for her, as well as an '80s classic.
I voted for "Love Will Never Do (Without You)". "Black Cat" was close but its sound gave it an added appeal for audiences that generally wouldn't listen to her music. "Love Will Never Do" was more of the same for Janet, and the last single of a hugely successful era. It was bound to be another hit (and it was) but I never saw it as anything more than just another good single. Had it been released ahead of something like "Escapade" or "Alright" then I'm sure more people would remember it today, and I'd be voting for "Black Cat" instead.