The Unforgettable Fire-The Joshua Tree-Rattle & Hum era was a groundbreaking era for U2 and for rock history as well
But maybe the Achtung Baby-Zooropa-Pop era has been more important
So as the first and definitive album of the most important era it has to be Achtung Baby
'All That You Cant Leave Behind' should get some consideration also
It has been a groundbraking moment for U2 and kickstarted the fourth era for U2