the 90s "were" or "was" the best decade

"the 90s were the best decade ever" or "the 90s was the best decade ever"

"the 90s" and "decade" are both referring to the same singular period of 10 years, so "was" seems to make more sense. "90s were" rolls off the tongue better though.

I feel that this may well be a duplicate, but I can't discover an original.

These Google Ngrams show that decades are used with singular or plural agreement. They're essentially collective nouns for the years they contain. At least in the UK, referring to the Thirties say (and notice that the noun is often regarded as a proper name) and having in mind the complete time interval would be regarded as looking at a single interval, and be used with a singular-form verb:

The Thirties was the time that saw the rise of Nazism.

Sometimes, proximity agreement sounds far more natural:

The Sixties were the best years of the 20th Century.

I agree that the more logical 'The Sixties was the best decade ever' sounds rather unnatural, so I'd rephrase.