If we're talking pitty-pat flurries, Taylor. For real, meaningful punches thrown from different angles, Camacho.
I would put Roy down as the fastest at career destruction with his incredible ability to get KTFO. Roys speed, which was not the fastest, came at a cost of zero fundamentals. Great reflexes, but that's it. Now he knocked out a lot. Camacho was fast. Khan has/had some speed.