I could mention a whole bunch of albums nobody's heard of, but the thread is "best underrated," and not "best unknown." So here's a few for you:
Candlemass--Chapter VI: I don't care if it doesn't sound like Ancient Dreams, it's a great melodic / doom hybrid CD
Accept--Russian Roulette: Everyone thought it sucked. I kept listening and it grew on me. Title track is amazing.
Memento Mori--Rhymes of Lunacy: Rhymes is NOT a Candlemass clone--clean out your ears!
Riot--Privilege of Power: Thundersteel was great, then no one cared about the follow up! Privilege of Power was also strong.
Running Wild--Pile of Skulls: Everyone talks about Death or Glory, but this one has a better production. Also has "Black Wings of Death"--possibly the ultimate RW song of all time.
Savatage--Hall of the Mountain King: Hall of the Mountain King will always be the ULTIMATE Savatage for me. Infectuous rhythms that make you want to smash things.
Skyclad--Jonah's Ark: I actually enjoyed this more than Burnt Offerings. Cleaner sound and more mature songs. Unheralded.
Black Sabbath--Headless Cross: I'm not the biggest Tony Martin fan, either. But Headless has some great songs, and was dismissed before it was even released. Unfair criticism does not impress me. At least listen to the material first.
Queesryche--The Warning: I hated this album because it wasn't as guitar-heavy as the EP. Then I kept listening, and it became one of my favorites of all time. I wish I knew the words the guy is speaking in the background of "Roads to Madness."
Kreator--Coma of Souls: Best production of any Kreator in that era. Yes, I like this more than Extreme Aggression.
Nocturnus--Thresholds: There, I said it--OK?!? Most unjustifiably trashed album of the '90s. Everyone called it "Cold Lake 2," but it was really an amazing stab at death/progressive. People couldn't understand it, so they killed it.
Samael--Blood Ritual: One of the best guitar sounds ever put on tape. Incredible groove for black metal.
Sinister--Diabolical Summoning: Can't undertand why people didn't like this. Intense speed/death blend. Unbelievable guitar work.
Venom--At War With Satan: Sure they were drunk out of their minds, but I could care less. Very....ehem...."loose" production values here, but I had never heard a 20 minute song before this, and it was just awesome. Some of the lyrics put me in stitches to this day.
That's enough for now
--S