It's kind of fun to pick the worst U2 songs, since there aren't very many of them -- there isn't much musical fat on their Irish bones.
Talking about post-Boy album tracks or A-sides, I'd have to go with "Red Light", "Miami", and "I'll Go Crazy if I Don't Turn This Crazy Song Off" -- one from each decade, all stinkers.
Runners Up: "A Celebration" and "Stranger in a Strange Land"
But still, none of those songs are absolutely God-awful. I mean, none are as bad as, say, the average Bon Jovi A-side.
Then, if we're gonna count pre-Boy stuff and B-sides, I'd have to name: "Jack In The Box" (was this ever recorded?) ; "Things To Make and Do" (the most boring U2 track ever?) ; "Touch" ; J. Swallow ; "Treasure" (loved by Paul McGuinness, despised by the band) ; and maybe their cover of "Paint It Black" (almost ruined the original for me).