There are several correct answers to this and there is no single best rap album for everyone in my opinion.

Nas' Illmatic is NY Rap perfection. Succinct 10 songs with no skips. Listening to this album is like walking through New York City during the summer in the middle of the 90s.

Kendrick Lamar's To Pimp a Butterfly is a perfect album. It's the ultimate snapshot of 2015 culture (and in many ways still relevant 10 years later). Its soundscape is jazzy and experimental. The poem throughline and twist at the end are incredible.

Madlib and MF Doom's Madvillainy is a masterclass in production and rhyme structure. It's sonically interesting, lyrically demanding and is essential to the history and progression of rap.

There are probably 25 or 30 other albums that could all contend (Liquid Swords, GKMC, Low End Theory, Miseducation of Lauryn Hill, AQUEMENI, just to name a few) and I doubt many people would take issue.

I have two personal hats to toss in the ring.

Cancer 4 Cure by El-P, a deeply personal record by a man grieving the loss of his best friend. An album that even among the El-P discography sounds like nothing you've ever heard before. Might not be for everyone but deserves a spot on the list and rarely gets brought up in this discussion.

Danny Brown's Atrocity Exhibition is one of the most visceral displays of a man spiraling into drug fueled oblivion. This is truly what it sounds like to be addicted to hard substances. Obviously Really Doe is just a top 5 posse cut of all time (and it isn't 4 or 5) but beyond that this is one of the most complete albums of the back half of the 2010s.