Achtung Baby, and it's not even close.
Boy carries a light post-punk feel through its runtime. Bono's faux British accent loses whatever edge songs like Out of Control and Twilight could've had. (I still love those songs, but they aren't very hard-hitting.) An Cat Dubh and The Electric Co. are the most threatening competition here.
Achtung Baby, on the other hand, features U2 at its most acidic with Acrobat, most industrial with Zoo Station and The Fly, and most honestly intense with the conclusion of Until the End Of the World. That heaviness continues even in the mournful Love Is Blindness, which shows U2 in a rare crossover between love and angst.