There's actually a few that could be listed. I think my quickest "fight ender" technique, which i've used with success on a handful of occasions, is an open hand thrust to the throat/adam's apple. I know it sounds cheesy, but i'm telling you it works every time.
Basically you throw it like a straight punch, but with your hand open like in a grabbing position, you slam it into their throat, hopefully making contact before your arm is completely extended and down they go holding their neck choking. BTW, i've never seriously hurt anyone with it either.