I think throughout the years every Led Zeppelin album was my FAVORITE Led Zeppelin album at one point or another. That's the greatest thing about them. Every album was a masterpiece and completely different from the others. I don't think there is another band that could change so much and still sound like themselves. Don't get me wrong, it's not like they were all over the musical map. They stayed based in the hard rock world. What they did do was blend in other genres like blues and folk. Specifically Renaissancian folk. Here they used some traditional American folk too. I was listening to a Blind Willie Johnson album a while ago and Jesus Make Up My Dying Bed came on and I said wait that sounds an awful lot like In My Time of Dying. Turns out because it is pretty much the same tune. Although the Zeppelin version is closer to Bob Dylan's version from his debut. Zeppelin for all of their excess had deep roots in music and listening to them and really kicking over the stones leads you on a long quest through the history of music.
And I didn't even mention the Indian chords of Kasmir, one of the very best Zeppelin tracks ever.