Gotta agree with the Gravediggaz & K. Jorge mentions. Kurious' "A Constipated Monkey" album gets lost in the mix when talking about all the absolute Hip Hop Classics of the 90's. Also, more recently is MF Grimm's "You Only Live Twice". This album was a top 12 of the year for me a couple years back and has gained Classic status, personally. Most anything by Masta Ace, especially "Disposable Arts" and "A Long Hot Summer". Amazing concept albums along with Prince Paul's "A Prince Among Thieves". Another off the top is Edan's "Primitive Plus" which often gets over shadowed by "Beauty and the Beat".
I love concept albums, so i'm a have to check out Aceyalone's "A Book of Human Language".