I think I just beat you all.
Nah, I met Christina Aguilera (before she was a total whore even). And she didn't come CLOSE to having that much makeup caked on. And she's tiny.
I've met, have had interesting discussions with, and have had photos (or been on video) with...
Sir Roger Penrose, Al Pacino, Robert Wald, James Hartle, Kip Thorne, Yehudi Menuhin, Stephen Hough, Werner Israel, Charles Misner, Carl Wiemann, and (Prince Edward).
I win.
The only person I even have heard of in that list is Al Pacino. I suppose Prince Edward would count too, but royalty doesn't really get you far in the States, so it's not really that impressive. 
Would count for what? In my opinion, Roger Penrose and Werner Israel are more impressive people than anyone else on the list. Penrose is widely accepted as the smartest person of the 20th/(21st) century, and after we're all dead he could well be thought of as in the same group as Newton, Leibniz or Einstein. Then, he will be remembered, and Prince Edward and Christina Aguilera will most likely not be.
Edit: Kip Thorne and Charles Misner co-wrote the bible of General Relativity alongside Wheeler, the father of quantum electrodynamics. Right now Kip Thorne has a 300 million dollar research grant to work on detecting gravitational waves, and is advising Spielberg in a new science fiction movie. About 21 years ago, Wald wrote a ground breaking book on General Relativity, that added many new conventions that are widely used, and has changed how a significant number of people do mathematical physics. Carl Wiemann is a nobel laureate in nuclear physics. Yehudi Menuhin is arguably the greatest violinist of the 20th century. Hartle has written a successful textbook in GR, and has collaborated with Hawking on many of the greatest achievements in modern physics.
I don't see how Christina Aguilera could top any one of those accomplishments
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