What is the best way to train for a fight? Not a contest, but a real fight in the real world. Like Socrates used to do before getting too far into his debates with the Sophists, let's define our terms:
1) The first definition that I have of "Fight" is being attacked without provocation. Ambushed, sneak-attacked, sucker-punched, approached in an alley and cornered, someone pulling a weapon on you or threatening you in some other way. This would also include those who work in a profession that requires you to engage in physical violence to maintain safety and order.
Numer #1 would clearly be self defense which is legal.
2)The second definition is the old "Monkey Dance" where two hot heads decide to throw down in the parking lot or back alley or school yard etc. and face off in a sort of fisticuff duel.
#2 Is a fight, which is definitely illegal. I would advice against doing anything illegal.
To me, the second one can be avoided (and has been avoided ever since I've had a job I can't afford to lose by engaging in a pissing contest). However, the best preparation for this type of encounter is most likely going to be sparring where two prepared opponents are squaring off and going at it.
The first scenario is different. Sparring will not prepare you for it. Training/drilling of techniques, training your body how to move effectively and preparing the proper mindset to break free of the "freeze" and shock that one gets into when one is unexpectedly attacked so that one can go berzerk when throwing the techniques that one has ingrained into one's reflexes until the conflict is over.