I'll have to go with UTEOTW. The Fly rocks but it's sort of dated due to its importance in establishing U2's new edgy style. UTEOTW is a song that exists in between all these rockers and ballads, hits and ready-for-arena anthems. It's a mixture of all of those and it's simply amazing. Lyrically i think it's one of Bono's most inspired works. Judas singing about Jesus. It's such an ingenious idea, and if you didn't realize it's true meaning, it would still ring true on another level.
"In waves of regret, in waves of joy...i reached out for the one i tried to destroy"...in the top 10 of greatest lines from a U2 song ever.