First and foremost, this album cements (to the extent it had not happened already) Led Zeppelin's place in history as the single most important band after the Beatles and the Rolling Stones. Sorry, it isn't even close. Just think about all the music, good and bad, that follows the trail first blazed by Led Zeppelin. Good: Van Halen with Dave. Bad: hair bands. Good/bad: Ozzy Osbourne; Van Halen with Sammy.
By the time Physical Graffiti was released, of course, Led Zeppelin's influence was already pretty well recognized. I'm not sure anyone was ready for this one, however. Epic track after epic track, played louder than anything before or since. Robert Plant's shriek reaching new heights. Jimmy Page's lead now fully formed. And the best rock rhythm section ever assembled, with the possible exception of Gibbons/Hill/Beard.
My personal favorite? In My Time of Dying. I didn't even know until years later (how many years later? Well, I learned it from Wikipedia) that this was an adaptation of a blues first recorded by Blind Willie Johnson.
Wait, maybe my favorite is The Rover. It's unbelievable that this track was recorded two years prior to the sessions that produced most of the songs on the album. A power trio + vocal performance by which all others must be judged. This wasn't good enough for Led Zeppelin IV?
No, my favorite (or at least the one that is the most impossible to get out of my head) has to be Ten Years Gone. The guitar solo positively swings.
I guess I can't decide which track is my favorite. I'll have to listen to the album some more.