I'd pick Tszyu if we are talking absolute prime, he fought a lot of slick, fast boxers and timed them with the right everytime. It's easy to forget how fast Tszyu was in his younger days also check out the mid to late parts of this vid.

http://www.youtube.com/watch?v=vqLhPutAl6Y

Manny is a freak though and looks terrific lately so who knows. Be a great fight though.