Problem with lists like these is that you end up debating between guys that have exceptionally fast hands but have plodding slow footwork or other guys that move all over the place but aren't really all that fast from an athletic standpoint.
In any case, when it comes to speed, Mighty Mouse is the first name that comes to mind.
"I think smoking pot is good for mixed martial artists. It's a new day and age, this is, uh, the year ... Fuck year is it? I don't know, because I've been training and smoking pot like I should..."