I've had a theory for a while that the best Rock song ever written is "Memphis, Egypt" by the Mekons. Trying to describe it in words is pointless, so go listen to it if you haven't heard it. It's on the album The Mekons Rock 'n Roll. I've had the privilege of hearing them play it live, and all I can say is, holy ****.
There are a few songs that I think are almost as good as "Memphis, Egypt". "Club Mekon", also by the Mekons, comes close, and actually follows immediately after "Memphis, Egypt" on the same album! This has to be the greatest two-song sequence ever recorded.
"The Headmaster Ritual" by The Smiths (on Meat is Murder) is possibly a better song than "Memphis, Egypt", but loses some points for the mechanical production that saps much of the energy from the track. (Aside: the best Smiths album is Hatful of Hollow because it is (mostly) a collection of tracks from radio shows, and captures the manic energy of the band much better than any of their studio output. But you knew that.)
I recently discovered on Pandora a song called "I Stare Out..." by The Verlaines which I think could be as good as all of the previously mentioned songs; at the moment, I consider it a major discovery. (Had you ever heard of The Verlaines? I hadn't.)