That honor obviously has to go to ATYCLB/HTDAAB. Those are two dreadful, heinous albums that an older U2 would have been ashamed of.
What?! I'm with you, to an extent, on HTDAAB (though it still has several excellent songs). But ATYCLB is a great album--not perfect, mind you, but a solid 4/5. It's the album that completed the narrative arc of their creative career (and that would have made a fine farewell, perish the thought, had they been so inclined).
For me it's either HTDAAB/NLOTH or NLOTH/SOI though, again, none of them are truly bad albums.