Yep, we're all gay in here. Anyway...
I also think it's a little tricky to say what MAKES the decade best. Honestly if you dropped all the albums from the past 40 years on the floor in front of someone who'd never heard any of them and had them listen to them without knowing what order they were released in, I'd bet dollars to donuts that the most recent music would be their picks for best.
It's understandable, as the years go by bands build upon what earlier bands did. They aren't pioneering shit or taking things into new territory (some are like Portal) but they're often perfecting it. Plus production is getting better as the years go by not in terms of budgets but just learning how to use the tools to achieve the sounds we want.
So when I say the 90s were the best decade, I don't just mean it's where the best albums are, I'm largely referring to how metal was forming and changing, plus how it was brought into the spotlight briefly. That's when metal went from this small grouping of half-formed subgenres and vague ideas into the gigantic, multi-faceted behemoth we know today.