Seriously. I'd possibly go with Like A Prayer as it was a single that stood her apart from the Kylie pap I thought the first few waves of her music was. Notable that Ciccone Youth kinda made her more credible too in '89. In the mid 90's I was at a rock party for some music studio, a band were just about to come on and "Secret" was played through the PA. Loads of punks and metalheads looking at each other with a what's this look (in a good way). I'm undecided but would go with Like A Prayer or Secret.
http://www.youtube.com/watch?v=79fzeNUqQbQ

http://www.youtube.com/watch?v=EPHUZenprKc