My favorite opening acts for U2 are the ones that I never got to see at a U2 concert, but have seen live (most of them) at their own headlining concert: Public Enemy (the one group I've never seen live), Kraftwerk (Paramount Theatre - Seattle 2003), Garbage (Pacific NW - Vancouver/Seattle/Portland, 2002 & 2012), & Florence & The Machine (Pacific NW - 2012). I wish I could've seen these acts open for U2 as those concerts would have been complete concerts from start to finish.