Unitron
I'm of the opposite opinion of most people when it comes to Zeppelin. You could make one great solid single-disc best of compilation with all the songs I like from Zep's self-titled albums and Houses of the Holy. Though, the latter is when the band was just starting to get good. More variation in songwriting, more experimental, and even more memorable hooks. Physical Graffiti though, that's where they take what that started and just went all out.
One of the few albums that absolutely warrants being two discs, two discs of some of the best hard rock ever put to record. Riff after riff, hook after hook, melody after melody, it never lets up with a weak moment. In My Time of Dying is how you make an 11-minute rock song, it has the energy and intensity of an extended live cut. The heavy raw blues riffing is just relentless, with several great hooks. Trampled Under Foot though, if I had to pick, this might be my favorite Zeppelin song. There's something about funky songs from heavy bands in the 70's that I can't resist, but this was the first of its kind I heard. The ballsiest foot-stomper out there, it's just such an irresistible groove.
The iconic Kashmir just may have had an influence on Rainbow for songs like Stargazer and Gates of Babylon, with its plodding Phrygian climb. The Rover, In the Light, and Ten Years Gone prelude some of the sounds of the following Presence with the balance of hard rock grooves and melancholic melody. Down by the Seaside is basically a chill country song, but not without an intense bridge. The album's finale of Sick Again is another heavy stomper, but there's parts where it sounds like this hard riff and colossal drums are being layered over themselves several times. Have no idea how it was produced, but whatever it is it sounds fantastic.
Even the songs that might at first seem like filler are great, Houses of the Holy I used to think was a weak moment, but now I love it for just being a great hard rock song. While the following Presence is my favorite Zeppelin album, Physical Graffiti isn't far behind at all. The greatest double album in rock as far as I'm concerned.