I watched this just a few days ago. It's pretty funny, ranks up there with the rap battle between Harry Potter and Voldemort.
Vader won by a landslide
It's a tough call really. Hitler had some great lines, "Step into my shower", but so did Vader. I think Vader force choking Hitler and moonwalking tipped it to his side.
I think vader shoudl force choke hitler after the rap battle