See what I wrote in the other U2 thread; I think... wait, I'll bring it over here, too...
U2 has a history of sorts with albums being up and down, so it's no surprise. It's like they put out an album and it kills, and they have the inspiration but maybe not the ideas and the next one flounders, but then they realize they have to knuckle down and the next one after that is a monster...
WAR
Unforgetable Fire
THE JOSHUA TREE
Rattle and Hum
ACHTUNG BABY! (My vote for their best album)
Zooropa
Pop - isn't great, but it was a vast improvement over Zooropa, and was of a longer period (the "Achtung-Zooropa-Pop" touring arc).
All That You Can't Leave Behind - isn't great, but was a more muted, reserved return after the glitz of Pop).
ATOMIC BOMB (Third best album, after AB! and TJT)
No Line...
SONGS OF INNOCENCE
Pop is the anomaly, because it's not for lack of ideas, or inspiration, it's lack of time. That really COULD have been a great album (the last song is one of their best ideas, but just... needed to gel. The other "lesser" records are the opposite. They put a LOT of time into No Line, and it's a mess. Zooropa would have been a better EP than an actual standalone album (the only song I REALLY like is the title track).