Too many to choose from, but top 5
Time from Dark Side
Wish You Were Here from WYWH
One of My Turns from The Wall
Waiting for the Worms from The Wall
Free Four from Obscured by Clouds
And honorable mention to Several Species of Small Furry Animals Gathered Together in a Cave and Grooving with a Pict, just for the fun it creates trying to get radio stations to play it.