Kwan's Fields of Gold takes the cake for me, but I also very much liked her Pocahontas.
Bourne and Kraatz had two very good ones, Return to Innocence by Enigma and Hotel California by the Eagles. I think it's been mentioned, but Grishuk and Platov's You'll See was very good as well.
Oh, and I forgot Khokhlova/Novitski's to Stop by Sam Brown. I think that was in 2008.
I don't think I ever cried during an exhibition program until Michelle's Fields of Gold. It didn't help that she was crying as well. I love that song, I love Eva Cassidy (she died wayyyy too young) and it was perfect!!!
Two fun ones from the Red Square gala in 2006: Brian Joubert doing his Daft Punk EX (which he got a lot of mileage out of over the years) and Alexander Abt skating to Gasolina (random Youtube commenter: "he really CAN skate to ANY music").
Serious Joubert: I was familiar with this one before, but I know his Hallelujah EX surprised a lot of people at 2009 Worlds as it was seen as pretty atypical for him.
And finally, Stephane Lambiel's first appearance on the world stage
Primavera Porteña - Daisuke Takahashi
Probably Yuzuru Hanyu's - Hana ni Nare (at an ice show if that counts). It's nice to see him loosen up a bit and connect to the lyrics personally.
Awwwwwwwwwwwwwww tiny wickle talented Stephane!!! Dat triple toe tho!
Agree on Akiko's Love Dance and Jason's Can't Touch This.
Older favorites: Anissina and Peizerat, Danse Mon Esmeralda. Gwendal singing along http://www.youtube.com/watch?v=ttIZfFetCwU
My all time favorite EX program--I wish we had more collaboration like this: http://www.youtube.com/watch?v=_N0_DeGtmLg
Johnny's 2006 Olys EX to My Way--there was a Japanese HD version, but IOC has blocked it http://www.youtube.com/watch?v=Ug7F_jI1yYY
At big events, like worlds or the olympics do all the skaters get to perform an exhibition piece? If not, how many are chosen and are they chosen based on placement or popularity, etc?
Top 5 usually. Galas at Worlds/Olympics sometimes have ring-ins - there were a few Russian juniors at the Sochi gala, for example.