It's definitely their most epic song, without a doubt. 23 minutes. It features some of the most beautiful playing I've ever heard, and the vocals perfectly compliment them. Mixed together they create an almost dream like atmosphere that's totally calm, yet wonderfully powerful.
Shine On (You Crazy Diamond) might be my favorite Pink Floyd, but then again there are so many songs that qualify.
I'm listening to the Meddle album right now. While it's definitely not their best album, it features some amazing compositions. Echoes obviously is the best song on the album, but I also find One of These Days and Fearless to be great songs. One of These Days has a truly menacing bassline that seems more Sabbath than Floyd.