Hardest for me.
Obviously I'm not going to touch ASOH, the best studio recording in U2's career. Nor am I going anywhere near a vote for UF or Bad. I don't agree with some people on this forum that the studio version of Bad is weak; when I first got into U2 at age eleven with the Best of 1980-1990, it was Bad and Pride that stuck out to me most as favourites. I've always loved the studio version, and then the live versions are just incredible - in terms of its entire performance career, I think Bad rather than Streets is the quintessential example of live U2.
So that leaves me with Pride or Wire. I prefer Pride in studio to Wire, but Pride's live versions, with only rare exceptions, have been mediocre and completely lacking the magic of the studio version. Wire, on the other hand, gained even more visceral energy live. I find myself at something of a stalemate. I'm tempted to abstain.