Without doubt Live Aid.
Nobody really understood what had just happened at that show until afterwards.
People can use the term 'pushing the envelope' too loosely these days,and back then I dont even think U2 knew what they'd just done in front of a worldwide audience.
Definately their most definitive moment in ensuring that they are still relevent today,24yrs after that event.
And the bit I like most about it,it wasn't planned,it wasn't staged,it was just a unique thing in history that U2 found themselves front and centre of.