Jon Bon Jovi and Richie Sambora's iconic acoustic performance of Livin' on a Prayer and Wanted Dead or Alive at the 1989 MTV Video Music Awards led to the creation of the MTV Unplugged series.
Now you can vote for that moment in the Most Iconic VMA Performance poll. You have until August 24 and I think you can cast a vote every few hours.
Michael Jackson's 1995 medley is currently leading the way by a healthy margin, though I'm not really sure if that was an iconic performance in itself or if it just looked cool and happened to be the King of Pop. (Someone who was old enough to remember, or has a better grasp of music history might be able to tell me.)
Anyway, vote for Bon Jovi. (But please don't be rude to MJ fans in the comments, it's a bit distasteful.)