In all serious though... It's hard to say M. Taylor was the fastest handspeed I've seen ever. Zab Judah was nearly as fast but I think had faster head movement and reaction time and just better footwork. Roy Jones is an obvious one although I'm not sure why that would be everyone's answer. Mayweather and Leonard were fast, but definitely not in the "fastest" category. Leonard was actually more quick on his feet than Mayweather. Camacho Sr. was a damn rabbit...could never catch the ****er in his prime. Fast ass combinations, bouncy as ****, awesome movement. He was an overall package as far as speed goes. Gamboa is worth a mention. He's outgunned everyone put in front of him. If it wasn't for his cocky ass attitude, he wouldn't be hit so much. Ali for a HW was pretty fast. About as fast as David Haye...that guy for a HW should get a mention...there's a lot. Although Haye obviously isn't in Ali's skillset or overall agility. He has that insane handspeed. There's so many more...those were just a few off the top of my head as first reactions.
RJJ for me - lightning fast in every aspect. Of the current fighters, aside from Pac, I have been impressed with the speed of Mike Oliver - quick little bugger.
Roy Jones, Jr. And I'm not talking P4P, I don't think anyone could hit a moving target faster than him or get out of the way of a punch faster.
Handspeed probably Howard Davis Jr. All round, on a P4P basis, probably Ali. Awesome speed of hand and fought while carrying 200 odd pounds around.