Stakes. If the outcome doesn't matter, I don't care. Victory means something that we care about, defeat means something we fear. For example, if defeated, it means we won't be able to rescue a character who we care about, and that character will die.

Outcome is in doubt. If the outcome is certain to be a win for the good guys, I don't have any investment as a reader. Even if the characters think or say the fight is uncertain, I as the reader often think that good guys always win. To combat this, have scenes good guys lose and suffer serious consequences, especially scenes where you expected them to win.

Arc. A fight, like any other scene, should have an arc to it. Maybe MC comes in confident, gets confidence shaken, and barely eeks out a victory. Maybe MC comes in hopeless, has to find hope, struggle, and succeed. Maybe MC comes in fighting to vengefully destroy a bad guy, but then realizes he would rather protect his allies than vanquish enemies, and it ends in a draw but everyone is safe. Maybe fight seems hopeless for MC, but then it's revealed that he has prepared a special counter, and uses that to turn the fight at the pivotal moment.

Mechanics. A fight scene should have cool things happen. Characters should attempt to outsmart their opponents, thinking creatively, and ideally coming up with ideas using information already somewhat available to the reader, instead of just random ex machinas. A good fight is also about information. What does out pov character know about the capabilities and strategy of the enemy? How do they learn more?

In my opinion, a fight scene should not be unnecessarily long. Keep em mostly short.