Outkast is one of my favorite groups, but The Chronic is the correct answer. Lots of other good ones already mentioned. I'd submit two more for consideration:
OutKast - Stankonia
Eminem - Eminem Show
Common - Finding Forever
Lupe Fiasco - The Cool
Nas - Hip Hop is Dead
There are some other good ones, some of which have been mentioned above. And this is just off the top of my head, so I may be missing a record I might put in there if I really thought about it. But these strike me as the top 5 of the decade.
Biggie's 'Ready to Die' is without question the best rap album in history. I was just listening to it tonight - for about the 4387th time - and literally every rap is sick.