I'd put Section.80 as a classic just because its a hell of a lot more "listenable" than a lot of other "classic" albums.
Good Kid.Maad City and To Pimp a Butterfly are classics as well.
DAMN! came out last week, I'm not going to declare it anything for a little while.
Again, Kendrick will have to rap for another decade or be killed before more of the dusty hip-hop fans who see themselves as the "gatekeepers of hip-hop" will allow him into a higher position in the GOAT category. It has nothing to do with him as an artist, they just like a thick layer of nostalgia before they give any artists their props.
I wouldn't say Kendrick has more classics than Nas.
That being said, a lot of the "hater" posters will talk about Kendrick's verses being "about nothing" and then praise Nas Album Done, or some other verse where he does his usual cyclical rapping about nothing