Some great classic suggestions on here, but I argue Eminem is not the greatest of all time. He is the most accessible of the greats.
He is probably the most skilled rapper ever to record music. His verbal dexterity, humour and lack of dense slang made him extremely accessible in a way that Raekwon or Ghostface Killah (both Wu-Tang) could never match. Rae and Ghost are my personal faves, but their brilliantly inventive and unusual slang can be impenetrable to outsiders so to speak.
But is he the greatest? I'd argue not, because for greatest we need criteria.
It's why I also discount Tupac who has been mentioned at least a dozen times in this thread. Pac was initially a social visionary and campaigner, then an icon after death, but he also released a lot of middling to poor music, filled with hate and recrimination but little in the way of quality . I loved Johnny J as a producer (RIP) but his music is sonically tinny at times - it wasn't all Dre beats.
For me that criteria again:
• Truly skilful with their rhyming ability
A great discography spanning a decade or more
• Valued and admired by both rappers and public
• Game changing in their interpretation of the art form
• taking the art worldwide in some way
• a great ear for a beat
For me there's only one name in contention.
Jay-Z.
From 1996 to 2021 and still able to cause a worldwide buzz when he appears on a record despite his age in a genre that doesn't reward longevity
He's become the Rolling Stones of an art form that was predicted to die out in ten years. He's brilliant on the mic, he always knows which producers to work with, his feature appearances are legendary, he has at least five classic albums, and he's still hungry to push the sounds of the art form forward