Houses of the Holy!
No, wait, IV!
No, wait, Physical Graffiti!
ARGH
It really shifts between those three daily. I mean, HOTH has The Song Remains the Same, Over the Hills and Far Away, No Quarter, and The Ocean; IV has Black Dog, Stairway, Going to California, and When the Levee Breaks; The entire disc one of Graffiti is amazing.
I just can't decide. I'll get back to you later...