There's a lot of strong opinions in this thread, i'm surprised there hasn't been an argument as of yet.
I'll give my 2 cents to some of the bands mentioned so far that i know of...
Sepultura - Arise
Definatelly NOT overrated. Sepultura's best album by a fucking country mile. Chaos A.D is good, but nowhere near the quality of Arise.
Metallica - Black Album
Is it overrated just because it was their album that made them popular? It is their most popular album...not their best...most popular. I can understand how it can be overplayed, but overrated? Never. I'll never get sick of that album.
Fear Factory - Obsolete
I'd have to agree here that Obsolete is an underrated album. That with Demanufacture are FF bests two albums. Demanufacture may get all the glory, but Obsolete goes to another level of awesomeness.
Blaze - Blood & Belief
An excellent return to form for Blaze in my opinion. A very underrated album. Proabably his best out of the three.
Iron Maiden - Somewhere In Time
Very much underrated album. Probably my 2nd favourite Maiden album, after 7th son (which is also NOT overrated). Wasted Years, Stranger In A Strange Land, Alexander The Great & Caught Somewhere In Time are some of Maiden's best songs.
Iron Maiden - The X-Factor
Greatly underrated. I've been a Blaze supporter since day 1 and I enjoyed the two albums he made with Maiden very much. I think the X-Factor and Virtual XI were Maiden's best albums since 7th Son. FOTD and NPFTD were rather average in my opinion, so it was great to here something different when Blaze joined.
"Anything by Nightwish, and the finnish metal scene in general is overrated"
I almost fell off my chair when i read that...in fact, i did. That is by far the most idiotic and false comment i've read on here for a long, long time. Pure lunacy. There are two countries in Europe where the best metal music comes from: Finland and Germany.
Judas Priest - Ram It Down
I tend to agree that this album is looked over. It has some kick ass songs that are right up their with their best of all time. I may be on my own here, but i'd also add Turbo to the list of underrated albums. I dont care if people say that it sounds different or mainstream or whatever. I rekon it sounds great and is one of my favourite Priest albums.
I really cant think of any bands/albums which could be considered overrated. Everyone has a different opinion, which they are entitled too....except for that Finland comment

I just cannot accept that opinion.
I am too just giving my opinions and you can disagree with them if you want, thats what this discussion is about.
If i had to mention one band that could be considered overrated...i would have to say...Kamelot. I've heard all their albums and i just dont see what it is that makes them more superior than others. Most of their songs follow the same structure, most of them bore me cos they are slow and prod along. Musically, slow verses where Khan is almost speaking the lyrics, and then mid-paced choruses...
I'll admit, they do have quite a few good/great songs where they play the entire song up-tempo, like it should be. But they are too few and far between, and Khan's voice does annoy me at times too. I'm sorry, but i cant see any evidence of Kamelot being "groundbreaking" in their genre. I must say, however, that i thought Epica was a very good album and they do derserve credit for that. And also about half of Karma.
hmmm, thats about it i think