That is a stupid question. Anything on any album (except for The Final Cut, which is only half perfect) is all equally the best song ever written by anybody. One of These Days is quite good, too.

Wish You Were Here is a great song, but I just watched the Roger Waters DVD and that version kinda put me off.

Mother is bloody awesome on the live version of The Wall (as is the rest of the album)

But The Gnome is pure genius.