Toss-up between "Shine On" and "Echoes" and "Comfortably Numb," if forced to choose. "Careful with that Axe, Eugene" and "Several Species..." and "Set the Controls for the Heart of the Sun" pre-Meddle.
Trivial note probably shared by several others...on the day of Syd's passing, I received my long-awaited "Pulse" DVD (my VHS copy is long worn out). One of the better concert DVDs/videos there is, and it still barely does justice to the live show. If you know someone you want to make a Floyd fan, show 'em Pulse :-)