The Classic: I guess "Heroes" is the obvious choice. It's a lot better known now than it was back in 1985 when he played it at LiveAid (in fact, I'd say that LiveAid performance had more to do with making the song his most famous and popular one than anything else). Which is a bit of a drawback: it's too obvious, and we all know it was played at LiveAid. If he were to reprise it, it would have to be with an updated arrangement very different from the LiveAid one. So instead, I'd go with Fame. If nothing else, the line "Fame, puts you there where things are hollow" might have given a few of the more self-aware celebrities there pause for thought.
The recent one: Everyone Says Hi is the catchiest song of recent times, and something the crowd could get into. Lyrics would have a certain poignancy to the occasion too.
Collaboration: U2, a medley of One and Absolute Beginners.
"We get smart too late and old too soon." - Mike Tyson