http://www.goldminemag.com/articles/true-5-star-albums-led-zeppelin-physical-graffiti
Here's a review from Martin Popoff who often says that not o my is Physical Graffiti the greatest Zeppelin album, it's also the greatest ROCK album!
Speaking of album covers, I can honestly say that I don't really like ANY of their album covers. LZ1 was cool, 2-4 are all lame, Houses was goofy as hell. PG was ok, Presence was lame, ITTHD was a cool concept if you even know what that concept is. Coda is lame. The live albums are lame. The 1990 Box set cover was kind of cool. Overall, for a band that was as big and epic as they were, they really had some really lame album covers.