I would say definitely "With Or Without You". It's by far the song you'll hear most often on the radio, or the song many people will know (at least the song as a song, without a title). Songs from ATYCLB and on don't really count in this list cause they're too fresh to be judged so don't mention Beautiful Day (although I agree it COULD someday be in this list).
The second most recognizable song could be "One". Definitely not Sunday Bloody Sunday, neither Pride, they are not that well known among younger people. WOWY is, however.
This conversation brought me to memory what had happened last year with my best friend. We were in his car and on the radio they played With Or Without You. He didn't really say anything during the song but he was listening carefully. After the song ended, I started talking about U2 and mentioned how "With Or Without You" is U2's most famous song and biggest hit (not chart-wise necessarily). And he goes to me "REALLY?????". Poor guy, up until that moment he really really loved the song but for some reason he had that impression that it was not one of U2's hits, not even a single maybe. He goes to me "This song is probably among my top 3 of all time but I couldn't really accept the fact because it seemed weird that I would like so much a song that never really was anything big". Well, that song was the song that brought U2 to the top of the singles charts for the first time and made them the world's most successful band at the time!