This is a really vague question, as "fighting game" could mean anything. Do you mean Tekken, Street Fighter II, or Streets Of Rage?
At the very least, a fighting game should be written around a fighting engine with you just calling what frames you want from Mecanim. That's certainly though not really a question that Spine can solve since whether it's Spine, or a 3D Model, MecAnim is Unity's mechanism for controlling which animation you want next.
As such there is no "best animation controller" for a fighting game. It all depends on what your game does/is.
You'll certainly be needing multi layered Mecanims.
This might be interesting reading for you:-
http://kalandorstudio.com/nerg/handling-complex-state-machines-mecanim/