I think Shao Kahn in MK3 was the toughest fight I had to go through. I had to try at least twenty times before I actually beat him. Each time I kept coming back I would look for weaknesses and learn Shao Kahn's method of fighting in order to defeat him.

I played Mortal Kombat 4 and I thought Shinnok and Quan Chi were garbage compared to the extremely difficult fights I had with Shao Kahn.

Mortal Kombat: Deadly Alliance was probably the easiest boss fight I had in any Mortal Kombat game.

I have not yet played against Onaga but I hope he is a pain in the ass so that I would have fun challenging him getting my ass kicked and then finally beating him.