Mos Def has two certified Hip-Hop classics in my book. Those albums are Black on Both Sides and his duo group album Black Star. That puts him among a rare, elite group of MCs that have more than one classic, as few rappers have even one classic.
After that, you've got The Ecstatic, which is an excellent album in my book, and I also find The New Danger to be an interesting experimental project with some standout songs (the Kanye produced "Sunshine" is one of his best songs). The only real dud to me was True Magic, which was heavily bootlegged, not even promoted, and sounded like an incomplete album.
Overall, that's a great body of work. It's why Mos Def is in my top 5 MCs all time.
Behold as my odes manifold on your rhymes
Two MCs can't occupy the same space at the same time
It's against the laws of physics
So weep as your sweet dreams break up like Eurythmics
Rap rejects, my tape deck ejects projectile
Whether Jew or gentile, I rank top percentile
Many styles, more powerful than gamma rays
My grammar pays
Like Carlos Santana plays
Black Magic Woman
So while you fuming
I'm consuming mango juice under Polaris
You're just embarrassed
Cause it's your "Last Tango in Paris"
And even after all my logic and my theory
I add a 'muthafucka' so you ignant niggas hear me
And you remember take notes
As I sow my rap oates
And for you biting zealots, here's a quote