There is no official "fastest rap" that Eminem sings. He himself can vary the speed at which he raps, and there is no official record of the top speeds he can hit when in full flow.
It is generally considered that the following songs are the fastest: 'Biterphobia', You Don't Know', 'Same Song and Dance', 'Forget About Dre', 'Despicable', 'What Color Is Soul' and T.I's 'That's All She Wrote' - featuring Eminem.
'Biterphobia' - an example of his earlier work - has the most consistently high speed rapping so it is arguably the fastest, but there are moments in some of the above mentioned songs where Eminem speeds up faster than this.
The following link to YouTube is a video featuring Dr. Dre, Busta Rhymes, 50 Cent and Eminem all taking part in a freestyle battle of who can rap the fastest. Eminem seems to be at his fastest here, but it can't be confirmed whether the track wasn't speed up slightly, despite no increase in the pitch. www.youtube.com/watch?v=m0s1CXgICCQ For more debate on Eminem's rapping and the fastest speeds, it is probably worthwhile registering on his website's community section. This forum would be more suited - and knowledgeable - to accurately suggest his fastest material. The site can be found here - www.eminem.com/forum/

Despicable nd his part in forever

No love is probably fastest

Despicable then biterphobia then part in forever then some parts in stay wide awake

Eminem what color is soul a lot faster than biterphobia

Its definitely Eminem's 2nd verse in T.I ft Eminem - All she wrote Hands Down!!!

Wrong All Of You!

Go to youtube and copy paste the fallowing or use the provided link

[2009] Dr.Dre Ft. Busta Rhymes , 50 Cent & Eminem - Freestyle Battle (Fastest Rap) [2009]

His piece in forever is very fast.

Anyone who thinks that his fastest raps are in the songs No Love, Drop the World, or We Made You hasn't heard very many Eminem songs. You can't call yourself a true fan if all you have heard is the songs that are on the radio. Not being mean, but come back when you have actually listened to songs that EVERYONE doesnt know. For example: You Don't Know, Same Song and Dance, or Forget About Dre'.

In Drop The World by Lil' Wayne

I think his rap in forever is fast

Yah, I'm thinking forever is the fastest, or we made you. Ol I think mockingbirds the slowest