Most favorite: Physical Graffiti. Ten Years Gone, House of the Holy, Black Country Woman. It was my first "headphone" album way back when. It's a nice composite of songs, old and new, on many pleasurable levels. I've often felt that it is also kind of a summary or a bookend to the previous 6.5 years of their music before they shifted to the next Zep level.
Least favorite: IV. No denying it's a significant album and I'm on board with that. When the Levee Breaks - the benchmark drum sound is fantastic. Essential classic rock songs, all eight of them. When craving to listen to some Zep though, I just don't reach for IV. I can recite the lyrics forwards and backwards to Stairway, Rock and Roll and Black Dog thanks to the geniuses at Clear Channel.