I guess I'd have to go with October, too.
A few years ago, Q magazine (UK) did one of their "buying guide" features, and it was on U2. One of the concessions they made was that there was very little disposable filler in the U2 catalogue, and I think this is quite right. Each album is quite good in its own right, and they all have at least 60% first-rate material (and some have 100%!).
While I do think Boy gets praised a bit too much in relation to how much October gets knocked, I will say October is the weakest album on balance. As has been noted countless times, it's Bono who is struggling the most on it; not only are his lyrics either too random or too simplistic (depending on the track), his voice still hasn't matured and in fact he's a bit off-key here and there, probably a sign that the record was just too rushed. Some of the arrangements are kind of weird (the guitar flourishes in "I Fall Down" do not work), and again indicate an album that was probably under-rehearsed and rushed.
I think, though, that much of the problem was simply the band's (particularly Bono's) approach. On Boy, they were easier to love because they were kids singing about childhood. But on October, they're kids singing about God and huge life issues, but doing so in a very unfocused, adolescent way, which is easy to dislike ("No one is blinder than he who will not see"!).
That's one reason why I think War is much better -- because they learned how to take their hugely ambitious approach (which is like instinct to them--they can't do without it) and mold it properly in songs that have the right dynamics to frame their approach. By comparison, October somehow exposed the band's weakness of having more ambition than the musical chops required to carry that out.
But anyway, since they were just kids and it's the inevitable "sophomore slump", October is still a very loveable "weakest album", but a weakest album it is.
Though not the weakest, Pop is probably the one I would rate most disappointing, simply because (a) there was such a long wait for it, and (b) it revealed them, for the first time, to be completely out-of-step with contemporary youth culture, trying to be cool (again, for the first time), and failing miserably.
Oh, I forgot the worst thing about October: the 4 unbelievably bad haircuts on the sleeve!