It depends on speed. Taunt when used on slow Pokémon doesn't go very fast, and when sucker punch is used on a Pokémon about to taunt, it depends on speed and sucker punch will fail, while taunt still works. If use by prankster, however, it will have priority. If used with prankster and the opposing Pokémon uses Priority, it depends on speed. If extreme speed is used against prankster taunt, extreme speed will go first.
Sucker punch is an unique move, as it goes first, but it only works if the target is ready to attack. However it will fail if the Pokémon is not ready to attack. Sucker Punch against sucker punch, it will fail. It will also fail if the target moves before the sucker punch. Therefore, making sucker punch the fastest, but if it is against prankster taunt, it depends on speed.
Hope this helps!