Yeah, I mean I think to a very real extent metal's improving as the years go by but the 90s were just when ALL those monolithic albums landed. Pick a genre!
Death: Deicide, Cryptopsy, Morbid Angel, Suffocation, Incantation, Immolation, Death.
Black: Mayhem, Darkthrone, Burzum, Beherit, Bathory, Emperor.
Thrash/groove: Megadeth (Rust in Peace), Slayer (Seasons), Pantera, Machine Head.
Grindcore: Napalm Death, Brutal Truth, Carcass, more I can't remember.
Sludge/stoner: Sleep, Crowbar, Eyehategod, Acid Bath, Buzzoven, Electric Wizard.
Nu (and fuck you, everyone loved this shit when it first landed): Korn, Slipknot, Deftones, System of a Down, Static-X.
Bands were either debuting with albums that changed the landscape forever or released the albums they'd best be remembered by. I mean, I'm constantly finding new shit these days that I listen to more than the above, but the 90s were when metal AS WE KNOW IT became solidified and mastered.
EDIT: can't forget bands like Neurosis, Melvins, the whole grunge movement, Godflesh at their peak, Type O Negative, the days when Marilyn Manson was terrifying soccer moms...