Hey I saw this thread and decided to sign up so I could try and answer the OP.
As far as fastest kicks go I have no idea but the other posters have cited both 60mph and 71mph so that covers that.
As for the strongest kick, I have seen Spinning back Kicks measured at 1500psi+ however suprizingly its not the strongest kick I've seen, that honor goes to the Gastrizein which is a kick found in Pankration, it looks similar to a front kick but utilizes the heel of the foot and generates 2000psi of power. It is a very quick kick too, as all front kicks are, and is extreely easy to utilize as you do not have to take your eyes off your opponent and you maintain good balance. So in summarry it is fast, accurate, esy to use and extremely powerful.
Hope this answers your question.
PS: By taking into account that its not just the kick but also the kicker, I think it would be safe to say that the strongest kick is either the Gastrizein the Muay Thai knee, or TKD's spinning back kick.
Note however, that although the most powerful kicks range from 1500psi to 2000psi the same numbers can be generated by Boxing Punches. So yeah, kicking isn't always the way to go.