Just found out about these, but they're pretty funny.

Epic Rap Battle, Billy Mays vs Ben Franklin

I love these and Action Figure Therapy.

The Einstein-Hawking one had some great lines.

Beethoven vs Bieber.

"I'm committing verbal murder in the major 3rd degree,
My name is BEETHOVEN motherfucker, maybe you've heard of me?
Not the ST Bernard version, I'm the real O-G,
You wanna trade blows? You can't even hit puberty!"

Hitler's line:
"You stink Vader, Your style smells something sour,
You need to wash up Dawg, here step in my Shower!"
(I admit, I laughed)

Everything Abe Lincoln says.
http://www.youtube.com/watch?v=zj2Zf9tlg2Y&feature=relmfu

You're forgetting the funniest part of that vid.
EEEEEEEEEEEEEEEEEEHHHHHHHHHHHHHPICRIPBTTLSUHHISSRY

The guys making them have a great sense of humour (I think)