I'm posting in c/movies but including tv shows, anime, comics, manga, etc.
Personally I think the final war rig sequence in Mad Max: Fury Road is the most impressive live-action fight I've ever seen. The practical effects and choreographing are incredible and the fight keeps moving along by having the stakes raised and characters dying, it doesn't meander.
In animation it's harder to say. Attack on Titan had a lot of really well animated action (it used be so good, goddammit). The battle in Shiganshina in season 3 is the best, the narrative weight is so strong, the characters all have really good moments, the stakes are really high and the production is incredible, animation, soundtrack, sound design, voice acting etc.
Mob Psycho has the most consistently incredible animation of anything I've ever seen, I think the group fight against the teleporting psychic in season 2 is my personal favourite, even if it's not the flashiest, it's really well directed and just such a cool fight, even though it's not that long.
The ChainsawMan manga has a lot of good fights, the Falling Devil arc is like my favourite arc of anything ever, but that's mainly because of the characters. The art is stunning, Fujimoto at his absolute peak, but the action is pretty straightforward. I mainly love it because it's Asa at her best as a character, and Asa is my favourite character of anything ever.
Wow it was way easier for me to choose a live action sequence than animated. Honestly there's so much lazy action in superhero slop that Mad Max stands out so, so much.
Without reviewing hundreds of movies the Subway Fight in The Matrix is my choice.
Alternately, the fight in Seven Samurai where the sword master is introduced.
Animated? The fight between Motoko and the puppet terrorist stands out, as it emphasizes a lot of the themes of the story; Invisible, super strong, and super-capable, Motoko completely and effortlessly defeats the puppet terrorist despite his installed martial arts abilities. He simply cannot meaningfully fight her, and when the fight is over she looks on with something like alien pity.
Oh that fight when Algren groks his sword training in The Last Samurai and defeats like four guys in the dark is really cool.
Oh, the fight in Pirates of the Carribean where Jack manipulate's Legolas' academic but inexperienced fencing to get to the other side of the room in an attempt to leave without hurting him. it says so much about who the characters really are. You get an early hint that Jack is much smarter, more capable, and more aware than his goofy persona suggests, and also of his weird humanism; He's no one's enemy, he just wants to go his own way. Plus that line "People arent' cargo, mate" is a great encapsulation of humanist philosophy, and you can extend it to anti-capitalism if you want.
Was going to mention the lobby shootout from Matrix 1 but I felt Jackie Chan and his stunts are still superior since it's all him.
Word. I don't think the Matrix would have happened without Jackie and other Hong Kong movie martial arts pioneers.