Canibus & Eminem have beef because Canibus rapped a whole song thru the view of Eminem's fictional character, "Stan". Eminem responded on "The Eminem Show".
A freestyle battle? No question, Eminem. He's one of the best freestylers out there and if you need proof he can roast people download the "1997 Rap Olympics Freestyles" and listen to what he says to the other people.
As far as written songs go, beef seems to bring out the best in Canibus (see "Second Round Knockout"). I still dont think he has a chance though, b/c Eminem is the exact same way.
People Dissing Eminem
"Whitey's Revenge" - Everlast
"Search 4 Bobby Fisher" - Dilated Peoples feat. Everlast
"Slim Anus Remix" - ICP feat. Twisted
"Aint Nuttin But a B---h Thang" - ICP
"Set the Record Straight" - ID-X feat. Eminems Mom
Eminem/D12 Dissing Others
"Quitter" - D12/Eminem (Everlast/Dilated Peoples Diss) (6min, 44sec version!)
"I Remember" - D12/Eminem (Everlast Diss)
"Girls" - D12/Eminem (Fred Durst/DJ Lethal Diss)
"I'm Just Marshall Mathers" - Eminem (ICP/Vanilla Ice/Pop Music Diss
"Say What You Say" - Eminem/Dr. Dre (Jermaine Dupri Diss)