Deadpool (Marvel) 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.
Joker (DC) is completely unpredictable, no matter what version.
Monkey D. Luffy (One Piece)
Pinkie Pie (MLP: FiM)
Because of the unpredictability of Killer B's (Naruto) movements, his opponents are easily overwhelmed with all of the varying swings, not even Sasuke nor Itachi could read his swordplay.
Bobobo's (Bobobo Bo-Bo Bobo) Bobobo Roulette makes it so that everyone is a target, even allies, and the consequences are unknown.
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.
Via the Crazy Fist technique, Goku (Dragon Ball) throws his opponents off guard by either acting like a crazy monkey...
...or a mad dog.
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.