You know, as simple as this thread is, it actually makes you think a lot. I mean, who's finisher actually LOOKS devastating?!? Most of them have good impact but have no realism to them whatsoever. Some, however, look as if they would do some real damage if ever used in a street fight. Those are the ones that I'm going to focus on.
First, Randy Orton's punt has to be on the list. If you just ran up to someone as they were getting off the ground and kicked them directly in the face, I doubt they would get up.
Secondly, I have to agree with the GTS. If you were able to hoist an opponent onto your shoulders and drop them onto your knee as your knee was rising up to strike them in the face, I doubt they'd be conscious afterwards.
Then, there's Chris Jericho's Codebreaker. It's quick, efficient, and definitely looks like it does damage.
Next, HHH's pedigree could do some serious damage. If you locked someone's arms behind their back and drilled their face into the ground using the force of your entire body, I don't think your opponent would get up anytime soon.
My favorite, however, is by a superstar that recently left the WWE. This person is probably one of the most unlikely choiceS and I'm sure it slipped most of your minds, but whenever they hit their finisher on their opponent, you can hear the crowds' "ooh's" and "ahh's" from a mile away. That person is none other than Victoria with her Widow's Peak. Now, I really can't tell if it would do REAL damage in a fight, but DAMN that shit looks and sounds painful. No diva has ever gotten up from it. Whenever Victoria hits it, you know it's lights out and game over.