Bobobo's (Bobobo Bo-Bo Bobo) Bobobo Roulette makes it so that everyone is a target, even allies, and the consequences are unknown.
Using Crazy Fist technique, Son Goku (Dragon Ball) throws his opponents off guard by either acting like a crazy monkey...
...or a mad dog.
No matter what version, The Joker (DC Comics) is completely unpredictable.
Harley Quinn (DC Comics)
Monkey D. Luffy (One Piece)
Yusuke Urameshi (Yu Yu Hakusho)
Wade Wilson/Deadpool (Marvel Comics) is so unpredictable that not even Taskmaster, who can copy any fighting technique can't follow any of Deadpools movements. And sometimes not even Deadpool himself can follow his own movements.
Pinkie Pie (My Little Pony: Friendship is Magic)
Captain Jack Sparrow (Pirates of the Caribbean)
Naruto Uzumaki (Naruto) holds the title of The Show-Off, Number One Unpredictable, Noisy Ninja.
Killer B/B's (Naruto) unpredictable movements allow him to overwhelm his opponents with all of the varying swings that not even Sharingan user like Sasuke or Itachi could read his Acrobat swordplay.
Macroy (A.R.Hicks Series) does not think, he just acts and will throw himself in the most dangerous situation to throw his opponents off. Couple that with his ability to control his muscle movement, he is very unpredictable.
As the legendary hero with the power to do anything, Creasion (Senyu) is completely unpredictable, usually doing things for no real reason at all such as beating up the heroes instead of the bad guys or changing the order of events so anything can or can't happen.