Number 9 (Beatles), though it's hardly even a song.
There are many more lyrical songs that after hearing just the first few notes, I can immediately recognize even if I have not heard them for many years. Like the first 6 notes of Aqualung (Jethro Tull). But if I were more knowledgeable, I bet one could come up with influences to show very few songs are all that unique. They're all re-using the same musical scale and the same rather small set of musical instruments (which nevertheless can be re-combined in a virtually infinite number of ways.) Otherwise it'd probably just be noise. We seem to expect a balance of familiar and new.
