Its a really, really tough call. I'm probably going to have to say The Wall, just because its so long and has so many great songs, but my two favorite PF songs (Wish You Were Here, and Welcome to the Machine) are both of of Wish You Were Here. My favorites off of The Wall include "Hey you", "Goodbye Blue Skies", and "In the Flesh". And I can't discount the quality of the album Dark Side of the Moon, either. Not when it has "Time" and "Brain Damage/Eclipse".

Actually, I just remembered that Shine on you Crazy Diamond is on Wish You Were Here, so that album wins by a tiny bit.

[/rant]