Dunno. Illmatic isn't classic simply because it's a really great album, it's classic because of its impact on hip hop. It was and probably still is the golden standard for the genre on multiple levels, it's one of the first rap albums with multiple "super" producers, flow wise Nas turned the page on an entire era, etc.
GKMC is interesting because it's arguably the only undeniable classic of the new-ish generation (Drake, Cole, etc). In terms of influence you can easily point to the west coast's re-emergence on the mainstream level after that album's release/success.
TPAB is a better album to me but it's too early to call it classic, and I'm not sure it'll have the same impact. I could see it going down as a classic album, but perhaps not a "hip hop classic." My argument would be that TPAB has had an undeniable impact on jazz's popularity, and heralded the later success of Kamasi Washington's The Epic, which has been a stunning success for a jazz album and has rightfully received nearly every accolade imaginable. But in terms of hip hop I don't believe the genre is going to move an inch towards any direction Kendrick took with the album. TPAB isn't the first jazz rap album, and most of them haven't changed the trajectory of the genre.
Perhaps you could argue that the social message of the album might have more impact than the instrumentation focus. Beyonce's album seems like it was clearly influenced by TPAB for instance, and if we see hip hop take a more clear stance on Black Lives Matter/activism perhaps Kendrick will get credit.
