According to Google:
Keith Liddell is a boxer, mathematician and author. He holds the record for the "fastest punch" in the Guinness World Records. The punch was registered at 45 miles per hour.
I'm not sure how true that is (I've heard tales of faster punches) but that's pretty impressive. I think I can punch faster, maybe, but probably not with the same power as Keith Liddell.
Assuming both A and B are boxing only (with gloves, no kicking or grappling), and both A and B are the same size exactly, same height, same weight, same reach, and both boxers have trained for X amount of time, then the lead straight jab is the fastest punch (and least powerful). The rear straight cross has the same delivery speed as a lead wide hook, but is more powerful than a jab and just as powerful as a hook. The rear wide hook is the most powerful (and least fastest). The more compact the hooks, the faster and more powerful they become. So again, the only thing it comes down to is body movement. Whoever moves around more will have the advantage.