As far as vast improvements go, I think first of "11 O'clock Tick Tock" and "Electric Co" from Under a Blood Red Sky -- their original recordings were fine, but onstage they became terrifying. Truly no comparison there. Also the two live songs on Wide Awake -- "Bad" and "A Sort of Homecoming" -- so much better than on UF. And of course the rescue mission they performed on "Please" resulted in a classic: its segue into "Streets" is one of this band's all-time moments.
Yes, "Bullet," "UTEOTW," "Walk On," and "Silver & Gold" are all awesome live, maybe even defintive, but they were pretty great as original recordings as well, so they don't really belong in this poll. And for those of you who suggested the live "Fly" over the glorious studio recording, I'm taking down names and will be around later to officially sign your certificates of disgrace.