Could Iron Fist beat Luke Cage in a fight?
I'll give the Netflix answer without weaving in the comics counterparts where the answer would be probably different.
In this case, Luke by a sizeable margin wins. Bushmaster showed Luke is one who can be taken with speed and punches he doesn't see coming, but can still easily hold his own, especially with the power upgrade seen in season 2.
Danny, in his Netflix iteration can't take Luke even with the fist and the martial arts. Luke is a street brawler who can take a punch with the Iron Fist.
Defenders showed him getting hit with one iron fist in slow motion to the jaw. That was still equivalent to a normal punch, so iron fist power punch is equal to hitting a normal person when used on Luke Cage.
Danny may have speed and martial arts (I'm not gonna go for the cheap shot about how poorly it's seen in the series) but Luke is now enhanced from the second bath in what made him Power Man in the first place (re: season 1).
It'll be a martial artist trying to take on a bigger stronger opponent. Even with two fists, his kicks mean nothing. Danny can't win. He'll tire out before Luke breaks a sweat.
As a martial artist myself, I can say that with certainty. He should have a lower center of gravity advantage etc., but with only two glowing fists doing relatively normal damage and your kicks doing nothing, no contest. Luke wins everytime.
That's my two cents.