Obviously Stairway to Heaven is one of, if not the best, they or anyone else ever made, but apart from that, I would vote for
"You don't have to go" on Houses of the Holy.
"Communication Breakdown" because it's just nuts.
"Black Dog" because it's demented.
"Dazed and Confused" because the guitar solo goes double time and the whole song is crazy.
"Squashed Fly (on the wall)"
O wait, that wasn't them.