The whole 90s era was his peak for lyrics, but if I had to narrow it down to one album, I'd have a pretty strong argument to make for Zooropa.
Zooropa and Pop are both very strong. Lemon isn't only near (or at) the top of their tracks for me, it's one of Bono's finest lyrics, about memory vs record, and just straight-up poetry about the commodity of seeing: "Man melts the sand so he can see the world outside", "he turns his money into light to look for her", etc.
And one album later there's another song that uses the death of Iris as a jumping-off point for something much deeper in Mofo, where you get "looking for a sound that's gonna drown out the world/looking for the father of my two little girls", which is as personal and vulnerable a couplet as he's ever come up with, followed by further soul-searching along those lines in Gone, maybe the greatest self-reflection I've seen on the ambition of becoming a rock star and wondering if the ends justified the means in terms of identity once you get to the top of the mountain.
And one final reference to Iris in Wake Up Dead Man, with:
Jesus
were you just around the corner
did you think to try and warn her
were you working on something new
if there's an order
in all of this disorder
is it like a tape recorder
can we rewind it just once more?
This basically closes the loop on Lemon, which looks at grief through the lens of technology and how we can replay the images of what we're lost, and here Bono is pleading a series of questions to the figure he prays to, now wanting to go beyond technology, using metaphysical means to reverse time so he can see his mother again. It's heartbreaking stuff, coming from a 35-36 year-old man who's a parent himself.