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.
The club fight from Ong Bak is great, all the flying elbows and knees make muay thai/boran such an interesting martial art to watch, just look at this. Same movie has an amazing chase scene too.
Another Tony Jaa movie, The Protector, has one of the most impressive one-take scenes ever - a fighting climb up a tower through several floors of opponents. The choreography is a bit simpler and slower than some of Jaa's other stuff since obviously the physical strain of having to do this after climbing several flights of stairs impacts what you can do, but it's still amazing.
Ghost in the Shell: Stand Alone Complex has an instance of a kind of action scene I wish there was more of - where it's several minutes of planning and people maneuvering into place, only to explode into just a few seconds of actual violence. Another amazing scene is the helicopter sequence from the 2nd season, I love how maneuverable the tachikomas are shown to be, gotta be one of my favorite sci-fi vehicle/mech designs. The music and sound design's also great here.
Patlabor 2's jet interception scene isn't exactly an action scene per se, but it's genuinely one of the tensest things I've watched, just on the edge of my seat watching a fucking air traffic control screen (not sure what the proper term is for that) and listening to people talking about ETAs and coordinates
The Good, The Bad, The Weird is an amazing South Korean action movie, basically a cowboy movie but in Japanese-occupied Manchuria. One of the final action scenes is this big cavalry chase through the desert set to a remix of Santa Esmeralda's version of Don't Let Me Be Misunderstood, just an absolute banger
. The guy riding against and right through a Japanese cavalry formation with his lever-action is just 
Waterloo and War and Peace, just for the sheer spectacle of the Soviet government being like "sure, you can have an entire infantry division to basically reenact an actual battle of the era, we'll even train them in Napoleonic-era drill!"
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