I partially agree with Tony, in that it's difficult to pick just one. But I still put "Rock the Casbah" above anything that I'm familiar with (and I listen to a <b>lot</b> of 80's music)... I guess just because I like it the best. It's easy to listen to a million times, it's fun... it brings a smile. I'm not sure how anyone can't like it, and I guess that's why it gets my vote.
Tony also lists a lot of really good stuff. I wouldn't say any of it is better than "Rock the Casbah", but still really good stuff. As far as picking one Michael Jackson song, I would probably go with "Beat It" or "Thriller" over "Billie Jean", but taste plays a huge part in this.