#3 K.O Punch - Big Show
This isn't necessarily a really bad finisher, it just looks terrible and would have been more impressive in the 80's. No one can deny that The Big Show would knock somebody out with a punch. His hands are HUGE! The Big Show left the WWE in 2007 to become a boxer and adopted this move when he returned.
The problem with the move lies in its slow speed. In order to not decapitate his opponent, Big Show would have to be careful with how fast he threw the punch. This move is also like the Samoan Spike in that wrestlers survive punches during the match but are knocked out by this one because it's a finisher.
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Because of his size and speed, he has a very limited move-set but he has had better finishers in the past. He used the Final Cut and the Showstopper as finishers and while they were also bad, they were somewhat believable.