I chose When The Levee Breaks - the combination of Page's vocals and bluesy harmonica, Page's piercing slide guitar licks, and the powerful Led Zeppelin rhythm section (especially John Bonham, who puts in one of his most powerful drumming performances) make this monster the perfect closing track for the band's epic fourth album. It's probably my favourite Led Zeppelin song, and that's saying something!
Having said that, Babe I'm Gonna Leave You is a more than worthy competitor, standing the first example of Page's use of light and shade as it switches gears from the gentle acoustic passages to the much more powerful electric sections with great skill. It's certainly one of my favourite songs from Led Zeppelin I.