Houses of the Holy with Physical Graffiti close behind. As many have said, The Rain Song pushes Houses just to the top for me.
Great story about that track - when Zep was going to be inducted into the RnR Hall of Fame, Jimmy Page called Joe Perry of Aerosmith to ask him if he would do the induction speech. Perry's response?
"I told him I'd do it if he taught me how to play 'The Rain Song.'"
The plagiarism stuff I've always found ridiculous. The band never hid those influnces or tried to pass the songs off as their own. It was done in homage to their blues heroes who inspired them. It's the fucking blues - that stuff is passed down and re-interpreted from generation to generation. If Led Zep were 'plagiarists' then a whole shit ton of other bands were too.