I'll start: Sepultura. I can't get around that the album I'm most fond of is Dante XXI. 

Maybe Soulside Journey by Darkthrone. I can't stand any of the albums after that.

Celtic Frost - Monotheist outstrips anything they recorded in the "classic" era.

Helloween - Walls of Jericho maybe? I don't know how odd that is.

don't know how unusual any of these picks would be but off the top of my head:

Slayer - South of Heaven

Pantera - Trendkill

Mastodon - The Hunter 

Most fans will point to any one of the four prior albums as the best. The Hunter was the point where Mastodon began to be received more lukewarmly. 

Alice in Chains - Black Gives Way to Blue

I believe the majority of fans have a problem with anyone who says that an album without Layne is the band's best. Most will say Dirt, Facelift or even the self-titled first. 

I don't think that's very odd. 

Odd would be someone saying Chameleon was their favourite. 

After Forever - Invisible Circles (Their second lowest rated album on RYM, and by far the lowest rated by my friends, lol)

Between the Buried and Me - Coma Ecliptic (Their least popular, but I love it, lol)

Evergrey - The Inner Circle ((it is considered among their best, but most people prefer either In Search of Truth or Recreation Day over it.)

Exodus - Shovel Headed Kill Machine (I think anything with Rob Dukes being a favorite would be considered odd, even though this is his best one.)

That one is probably tied for me with the debut, such a great album

I personally also like The Hunter best(though Crack the Skye is still a close fave).

An easy one for me would be Crimson Thunder by HammerFall. Always liked it FAR more than any of the previous 3. Hell, I even like the next album, Chapter V, more than those!

Within Temptation - The Unforgiving

I guess it depends on if you're more of an old school WT fan or not. I am, but this one is seen as the point they started to get more commercial, albeit not to the extent of future releases. It's my favourite though.

Symphony X - Twilight in Olympus

Yeah, I find it hard to believe I'm listing this one as odd and I don't know what everyone here's attitude toward it is, but when I was discovering Symphony X, this was also considered the lesser of what was considered the core four albums. It's high rated here, but the only one of the core four that hasn't passed an average of 4.0. And on RYM it's the only one of the core four that is not bolded. 

And one which is definitely odd:

Eluveitie - Evocation I - The Arcane Dominion

Eluveitie is one of those bands that have always largely left me unconvinced. But I really like their first non-metal album! 

Razor - Custom Killing

Annihilator - Set the World on Fire

There's probably more, I tend to love unpopular albums, but these are the first that came to mind that are my favorite album by the respective artist.

There's these as well, though they're tied with albums that aren't as odd

Judas Priest - Point of Entry (tied with Screaming for Vengeance)

Metallica - Load (tied with Ride the Lightning)

I actually love this album, for me it was a return to form!

Is this an oddity though? I think Monotheist is actually well regarded amongst the fans (and actually my favorite)

I'm weird in that I like them more in general with Derrick than with Max. 

To me, Max Cavalera has always come across as one of metal's most overhyped individuals; a man whose shadow has hung over Sepultura for years since his departure and it feels like it's only with the last couple of albums that they've started to gain some respect back. 

When you look at the massive influence the band had it's always seemed To Mega Therion or Into The Pandemonium, or even Morbid Tales were very important. The biggest oddity was probably this band producing this album so long after their supposed heyday.

Opeth - Watershed (Ghost Reveries or Blackwater Park are the ones I suspect to be at the top of most people's list - both great albums of course).

Van Halen - 1984 (not sure how controvercial that is but the debut would be the one that most would go for I think).

It's a very good album, to be honest. I would love to see Gillan and Iommi collaborating again.

My choice is Voivod's Angel Rat. It's not THE best, but it's easily my top three from these canadians.

Elvenking - Two Tragedy Poets (by virtue of it being their non-metal album)

Helloween - Gambling with the Devil

Mastodon - The Hunter

Nile - Ithyphallic (tends to be lower rated)

Lacuna Coil - In a Reverie

Alice in Chains - Black Gives Way to Blue

Sepultura - Dante XXI