Essential Albums from the 90's for me:
Public Enemy - Fear of a Black Planet
My Bloody Valentine - Loveless
Slint - Spiderland
The Jesus Lizard - Goat
Alice in Chains - Dirt
Dinosaur Jr. - Where You Been
Nas- Illmatic
Radiohead - The Bends
Beck - Odelay
DJ Shadow - Endtroducing...
Jay-Z - Reasonable Doubt
Weezer - Pinkerton
At the Drive-In - In/Casino/Out
Neutral Milk Hotel - In the Aeroplane Over the Sea
The Roots - Things Fall Apart
The White Stripes - s/t
Then, we have the quintessential:
Nirvana - Entire discography
Pearl Jam - Entire discography
Sonic Youth - Dirty
Outkast - Entire discography
Tupac - Entire discography
Some already mentioned albums:
Radiohead - OK Computer
Smashing Pumpkins - Siamese Dream
I'm not going to go into nostalgic albums like Linkin Park - Hybrid Theory, because they are only good for their nostalgia factor.