Though I love 'Crashed Car' and 'Zooropa' (they're up there with the band's most brilliant work ever), I had to go with, perhaps, the most beautiful ballad U2 have ever written (I don't really consider 'One' or 'With or Without You' ballads), which of course is 'Stay'.
I adore the lyrical ambiguity of 'Stay' - how they cleverly seem so obvious when in fact there is a real dark undercurrent to them. Most people interpret the last lines to mean the "bang and the clatter" of the Angel (the narrator) "hitting the ground," when they could just as easily mean the bang and the clatter of the woman (the physical abuse of the "vampire" as the "victim") being portrayed throughout the song. The last lines send a chill up my spine for all the wrong reasons...
Three o'clock in the morning
It's quiet and there's no one around
Just the bang and the clatter
As an angel runs to ground
Just the bang
And the clatter
As an angel
Hits the ground