I voted for The Dark Side of the Moon - absolutely brilliant from start to finish and one of my two all-time favourite albums alongside Wish You Were Here.
I love Led Zeppelin I, which is an excellent debut album and has several of their best songs (Dazed And Confused being a particular standout), but I only consider it my fourth favourite Led Zep album after IV, Physical Graffiti and II and therefore it's a fairly comfortable win for Dark Side overall in my book.