And Simba.

WHY DO U ALWAYS SAY "WAS" mufasa IS the best fighter and he will always be! doesn;t matter if he died

^
The reason I use "was" is because, like you said, a few of the characters on this list are dead.

She is a killer...

Nala's a beast.

Kovu because he learnt to fight very well from his mother Zira

and Simba.

Nala, hands down. No competition when taking into consideration the facts.
Simba is pretty weak, as even scrawny little Zira was able to throw him down. The only reason he won against Scar is because he kicked him over a cliff. Even then he didn’t kill him, the hyenas did.
Sarabi didn’t get much air time as a fighter, she just blended in with the other lionesses.
Scar is good, but then again he went up against a weak lion. He himself even admitted he had no braun and wasn’t a good fighter.
Zira is also good, but she was too far gone. Her insanity led to her being taken down by a cub. Had she been able to control her mentality better and use it to her advantage, then no doubt she would’ve been unstoppable. She makes it to third on my list.
Vitani is like Sarabi in the fact that she didn’t have much airtime fighting, just her herself, as she too blended in with the rest of the lionesses.
Nuka... It’s self explanatory. The reason he wasn’t chosen as the heir. He’s scrawny, and weak.
Kovu was knocked out within seconds in his first fight (from what we see) by a lioness. His own sister. If a kick to head was enough to send him flying, God forbid he ever get smacked by Mufasa.
(Yes I know he’s dead, it’s an example).
The only other lion fit for best fighter is Mufasa. He was a huge, massive, powerful lion. No doubt he could take anyone on this list. So, then why is Nala my pick? Because Nala is a lioness, a female, who was able to hold her own and take down a male lion in a 1v1. She showed potential as a cub, and it sure stuck with her. She’s a very powerful lioness, she definitely earned her rightful place as queen. She’s fearless, and will take on anyone who challenges her, giving a good name to lionesses. That, is why, Nala is by far the best fighter.