U2 have spent the vast majority of their career as the biggest rock & roll band in the world, a title that captures not only their popularity but their importance. Alone among all the groups to emerge from the post-punk era, U2 channelled their yen for moody, experimental aural textures into clearly defined rock anthems and ballads -- the kind of songs that fill arenas yet still seem personal. Much of that sense of intimacy can be attributed to Bono, a lead singer who gravitates toward grand gestures yet remains grounded by his belief in humanity and the revolutionary power of rock & roll. This sense of righteousness has never abandoned U2, not even after the group sold millions of albums all over the globe, but it burned brightest on their earliest records such as 1983's galvanizing War, when the cavernous guitar of the Edge still seemed flinty and the rhythm section of Larry Mullen, Jr. and Adam Clayton conveyed the grit of the group's punk beginnings.