Mine: Burzum's Filosofem is one of the worst albums ever made (in black metal or otherwise) and the rest of Varg's work is not much better. 

Not unpopular with me, but definitely is in the wider world.

Hmm... Let's go with:

The Number of The Beast is way overrated and is one of Iron Maiden's weaker albums.

I may have said the same once, with the caveat that I still really liked it all the same.

Over time though, I've come to regard it fully up to its hype. 

Most people here know my feelings on Slayer, but that's probably my most unpopular opinion regarding metal: Slayer really isn't all that great. Granted, my experience with Slayer stretches to only a few albums and I have some small appreciation for certain songs, but that's just my point: the really great, memorable songs are few and far between. Reign in Blood only has two of them. 

Max Cavalera is one of metal's most overrated figures. I mentioned this recently in another thread, but I'll reiterate it here. His shadow has hung over Sepultura for years even when Igor was still in the band, causing everything they released to just be written off as 'not Max. Meh' for years. Max Cavalera, whose last album with them was fucking Roots, which as a secondary part of this unpopular opinion, is a pretty terrible record. 

One I'm not as strong about having only just started to form it, but: Megadeth plats heavy metal better than they do thrash metal. Going through their discography at the moment, trying to properly get some experience with the band at last, and after listening to Countdown to Extinction and Youthanasia I feel pretty strong than these are stronger albums than the earlier ones. 

Eluveitie's best album (that I've heard) isn't one of their metal ones. They were quite prominent for a folk metal act in the late 2000s/early 2010s, but I never really felt the hype. Then they made Evocation I: The Arcane Dominion, a pure folk album, and it was really good. They always did need to let Anna Murphy sing more and I also can't bring myself to give a toss about that band now's she's left them. 

Yes yes YES. Derek Green (and even other brazilian vocalists, like Claustrofobia's Marcus D'angelo) are way better than Max. Derek only got better over the years! And Roots is the most overrated metal album EVER!

My unpopular metal opinion: Pantera is another ultra overrated band. Phil Anselmo is a great vocalist, Dimebag was a great guitar player but Pantera never really excited me. 

Guns'N'Roses, good band but not great, too much pretentiousness that didn't deliver.

And finally, Dream Theater is NOT, but any stretch, the best prog metal out there.

Modern death metal is unlistenable bollocks.

I was listening to an old Pungent Stench song this morning, and it had an infectious groove (yeah, pun intended) running through it. In modern death metal however, that groove is missing. 

Instead, all the focus is on brutality and technicality, and it's just a fucking chore to listen to. OK, you can play 11/4 polyrhythmic scales or whatever, I'm impressed, but please write a fucking SONG! I simply can't get into bands like Ulcerate because of this.

Bring the fucking music back to death metal!

(And no, not by going "melodic". That's another whine for another day!)

My unpopular opinion is that death metal & grindcore growls are awful.  Black metal style harsh vocals are so much better.  Same with whatever Aaron Turner's singing style is called.

I'm probably the opposite. I find screechy black metal vocals highly off-putting. It's probably because I started with death metal.

Iron Maiden is overrated, Somewhere in Time was their last great album (and best album), and Paul Di'Anno > Bruce Dickinson.

I've always just left the guitar player hype thing to musicians. I have no clue how good or bad they are, I just go by what sounds good! And while he might well have been a musical prodigy, those first two Ozzy albums he was on just aren't as good as most of Ozzy-era Sabbath or any Dio-era Sabbath, and I've never thought to mention it out loud before.

This, totally. 

I think he would grow up more as a guitar player over the years, but we will never know 

That's what I found recently looking at the old Black Sabbath albums. There were good songs, but the albums didn't necessarily work well as albums. Metal has always been album music to me, rather than just a collection of singles or songs, and Judas Priest seemed to get that. I think Overkill and Bomber by Motorhead also work well.