All right, I'll play...
Boy :
"Stories for Boys" should have been a single. It aged really well and could very well be considered as "punk rock". Bummer it has not been played live since... what ? 1982 ?
October :
I would nominate "Rejoice" as a potential single. IMO the tune is stronger than "Fire", although the band probably wanted to show a different side of their craft on their 2nd album. "Rejoice" sounds as if it could be on Boy.
War :
I honestly don't think they could have done better than releasing Sunday Bloody Sunday (worldwide) and New Year's Day as singles.
The Unforgettable Fire :
There's this alternate mix of "A Sort of Homecoming" from Daniel Lanois and Peter Gabriel but I don't think it would have had much of an impact on the charts.
The Joshua Tree :
Not changing a thing.
Rattle and Hum :
IMO, "Heartland" should have been a hit and one of U2's signature songs. With a tighter mix, I believe it could have had some airplay.
Achtung Baby :
Not changing a thing.
Zooropa :
Not changing a thing.
Pop :
Not changing a thing.
All that you can't leave behind :
Release "In a little while" instead of "Elevation" and make it a classic U2 song.
How to dismantle an atomic bomb :
Not much to save, I'm afraid. Vertigo is a decent single but that's pretty much it.
No Line on the Horizon :
"Magnificent" should have been the first single, followed by an edit mix of "Moment of Surrender" and "No Line on the Horizon".
Songs of Innocence :
Not changing a thing. Not that i think the album or marketing were perfect but, in retrospect I believe they chose the tunes they had to.
Songs of Experience :
As much as I love the album... I don't think there's a song strong enough in there to qualify among U2's best singles. There are songs that I absolutely love but they do not carry the "U2 Masterpiece" mark.