The Who - I never heard any of this band's lesser albums!!! I'll get on that post-waste! Of the ones I heard, the weakest is My Generation, which I still really like (but the band didn't--they tore it apart in interviews in 1965!)
The Fall - Only heard 1979-85 LPs, but of those, the worst was Live At The Witch Trials, a huge disappointment except for the classic "No Xmas For John Quays."
Pearl Jam - Going through these guys' discography right now. Weakest is the 2006 self titled album.
Neil Young - I've only heard like seven of his albums. I don't remember much about Tonight's The Night but that may be a bad example. Harvest and Neil Young (1969) were just okay, but not bad. I never went anywhere near the 80s stuff that got him in trouble. Isn't Arc supposed to be really bad?
Flaming Lips - Probably something from the '80s, keeping in mind I've never really bothered with their whacko experiments like that 24 hour song. Of their classic run, Hit To Death In The Future Head peters out in the middle.
Elvis Costello - Didn't care much for King Of America, but again, this is one of those artists whose huge back catalogue has left like 20 albums unheard.
Metallica - Lulu, I guess, though I think that one was more Lou Reed's album and I only listened to it in passing like three times. The 1996-97 albums are not great, but they're not horrible, just overlong and lackluster. St. Anger is horrible, but I like parts of the title track and "Frantic," so that would place it ahead of Lulu.
Black Sabbath - I hate to keep giving the same answer but I haven't bothered with this band's disasters--I stopped after 1975 with full albums. I didn't hear Sabbath Bloody Sabbath, but have always been slightly unimpressed with Master Of Reality, which disappoints me because most people love that one.
Alice Cooper - I didn't hear the post-1986 disasters and have not even heard his full 70s catalogue anyway, or his first two albums. Out of that, I guess Special Forces is weakest, but isn't bad--just a bit slight. Joe reminded me of the existence of Along Came A Spider, which is possibly the single most nondescript mainstream hard rock album I have ever heard in my life, so go with that one.