My personal favorite is their third. All the acoustic songs are completely amazing....actually every song on the album is badass. Anyway, other opinions?
If I had to choose just one, it would be Physical Graffiti. 4 and 2 are right behind it. I really love all their music, which I can't say about any other band. Also if you haven't heard it, this is an amazing symphonic tribute to Led Zeppelin.