Side-tracking away from Maiden,
Some of the Ozzy albums mentioned earlier, Bark at the Moon and Ultimate Sin - I wholeheartedly disagree with anyone and everyone who says those albums are bad. They are by far better than anything Ozzy released with Zakk Wylde imo. Jake E Lee remains an incredibly underrated guitarist and songwriter - he had a tough act to follow of course and had it been Zakk instead of him people would have most likely reacted the same way.
In any case, the 'glam' era of Ozzy if you want to call it that was great. It is the albums that followed, No Rest for the Wicked especially that is imo the worst Ozzy album. I can't really say anything bad about No More Tears, it falls a bit flat at times but it also has more noteworthy songs than anything No Rest gave us on which there was almost nothing interesting.
As far as the worst metal albums - it's a moot question because every metal band under the sun has released some less than stellar albums. No band is ever immune from releasing a bad album and examples are plentiful in every sub-genre of metal.
However, some of the albums which my ears disagree with:
KISS - Unmasked: say what you will about Elder say what you will about 80s KISS, even Dynasty had it moments, but Unmasked for me unlistenable;
Virgin Steele - Life Among the Ruins: to be fair this was not intended as a Virgin Steele release but that's irrelevant since it was released under the band's name. This album is horrible through and through. I love glam as much as anyone else but come on, Virgin Steele is a band that play epic power metal operas; this is beyond dumbing down the band's sound, this is the late 80s / early 90s own St. Anger imo.
Savatage - Dead Winter Dead and Wake of Magellan ... simply fail to capture the true potential 'tage had back in the 80s and early 90s. Though by no means horrible or unlistenable, I think they were better destined for another band.
Quiet Riot's Self-Titled - is this even Quiet Riot anymore? In any case, the only decent song on here is "The Joker" has a nice groove to it but that's about it. The rest is just another mediocre band... Kevin DuBrow (rip) was right about why Quiet Riot never made it big unlike other similar bands of the 80s - they simply did not have the songs ... the good strong hits were far and few in between.
Def Leppard - Adrenalize ... I love 80s Leppard so much so that I think Pyromania is one of the greatest glam/hard rock albums of all times but by Adrenalize the band's sound became so watered down and whatever worked on Hysteria now sounded tired and cliched. If anything, at best this album sounds like b-sides throw outs from the Hysteria sessions but even that may be a bit too generous. To be fair, 'Lets Get Rocked' could have easily been part of Hysteria (and could have easily replaced some of the less interesting ballady songs on there) but the rest of the songs simply miss the mark. What followed was only progressively worse to the point that I only worship leppard for their 80s stuff.
Van Halen ... humm what to choose ... see the easiest target is III but I would say the worst is a little DLR number called ... DIVER DOWN... GAAWWDD why even record this?
I get the comedic tongue-in-cheek value of their twisted version of Pretty Woman (that was a cool video in a style we would see more of from DLR's solo career) but the rest? Admittedly the band was not too into this album either, it was recorded and released more under the pressure of the record company...
As for the Van Hagar era - guilty pleasure there , I won't blast When It's Love on full volume even though I totally love the chorus, but it is far better experienced with headphones on lol ....
Well this list could go on for a while but time as always is not on my side. All in all, every band is guilty or will be guilty of releasing some horrid albums. However, what is bad and how bad it is, is ultimately subjective ... unless we're talking about Risk and St. Anger which stand for the definition of objectively and universally BAD.