90's but admittedly I am biased because I was a teen in the 90's.
STP, Alice in Chains, Nirvana, Tool, Pearl Jam, Red Hot Chili Peppers, and Soundgarden are the best from that era.
Tied for 2nd best would be the 60's/70's.
Zeppelin, Black Sabbath, The Doors, Jimmy Hendrix, Johnny Cash, Merle Haggard, Pink Floyd, Tom Petty, David Bowie so many good ones from these era's almost impossible to list em all!
I would say the sixties and seventies had the best selections. However each generation I think has some really good music here and there.
The 90s had some really good tunes come out for a while so I don't know what happened once it entered the new Millenia. I blame Auto-Tune and corporate data inspired templates. Music today for the most part is just abysmal.