I honestly find that the beat to say you will (originally off Kanye West's latest album '808s And Heartbrake') brings out everything in a rapper. Literally, when rappers freestyle or do a cover over this beat, I find they let everything out. It is said that Drake really exploded in his version of Say You Will (off the 'So Far Gone' mixtape), and even Kanye's original version brought out a whole new side of the rapper. I recently found a Trey Songz cover to it, and it really is great and worth a listen.
Say What's Real - Trey Songz