If you're familiar with 50 Cent on even the most basic level you recognize that he is a troll instigator and also views everything through the lens of money and/or success. His shots at Jay-Z in that video expose what his agenda was. He's bigging up his man, I get it. But he's also conflating success with being "the best."
What is Eminem the best at? He was once a great technical rapper but his style is largely mined from Masta Ace and G Rap. Is anyone going to seriously argue Eminem is a better rapper than G Rap? Go ahead so I can laugh at you. 50 is from Queens, he knows better.
Let's look at albums. Does Eminem have a classic album? And if so does he have one that stands up well against the best work of just about any rapper who is considered great? I'm sure there are some 20 year old white people on this forum who would love to wax poetic about why the Marshall Mathers LP is superior to Illmatic or Ready To Die but people who actually listen to hip hop know better.
What about lyrics. Are his similes, methaphors, or storytelling comparable to the greats? Again I'm sure someone here will argue Eminem has surpassed Nas on each of these levels but once again, people who actually listen to hip hop know better.
If Eminem wins any contest it would be the "90s rapper who fell off the hardest" award, and even then he might just share the trophy with DMX (or Prodigy...). Listen to his music today. Yelly, off beat, lyrically embarrassing. He's so obsessed with the the technical aspect of his flow that the beat and tone of the record don't matter to him. It's like listening to someone try to cram a dictionary into a 10 page picture book; after all, what better analogy is there for the cookie cutter production he selects than a picture book.
I really liked Eminem when I was a kid in 1999. But the idea of listening to a man in his 40s rap about raping celebs, pooping on people, etc in 2015 has no appeal to me. Nor do his meaningless, generic "if you work hard you can achieve it" anthems move me. Dude is trash today, admit it and grow the fuck up.