It's really hard to say which album is the "best ever"...because while Dark Side of the Moon might be the best album in the eyes of a classic rock listener, it's not the best in the eyes of a rap listener.
Oh yeah, and best album, IMO, has to go to a legendary band. I'm talking Beatles, Stones, etc. I know Steely Dan and Deep Purple or whatever are great, but IMO they aren't deserving of "best album" because their albums pale in comparison with some of the great Stones, Beatles, etc. albums.
Classic Rock = Led Zeppelin IV
Pop = Michael Jackson Thriller
Hip Hop = Outkast ATLiens
are my picks