In choosing a representative song for a tour, the album it was supporting becomes a little less important to me than the stage design/atmospherics/etc. itself. If the band is throwing its touring weight behind the new album (Zoo TV being the best example), then, yeah, I'll pick a latest-album track. But when the band clearly has doubts about the new material (as with PopMart and 360), then I don't feel so bound.
For 360, I've got to give "Ultraviolet" the nod due to a) its visuals (including not just the laser show and the mic, but the "light bulb" at the top of the 360 set lighting up and starting its circular progression of hitting the whole stadium right after Bono sings, "Your love was a light bulb hanging over my head" -- at least in DC), and b) the way it starts the encore that encapsulates the major themes of the tour.
Runners-up: "Crazy Tonight" for getting all band members around the stage and "Your Blue Room"/"In a Little While" for the space imagery and astronaut video that arguably most get at the space setting of the tour. And then I'd have to say TUF for its incredible beauty . . .