Shadow of the Ashen, from souls clone Ashen, popped into my head immediately. The intro, the arena... Just builds tension and atmosphere. Second phase is awesome too.
Sans - Undertale
memeworthy? yes. a legitimately hard boss fight that feels rewarding to finally beat? also yes.
Nightmare King Grimm - Hollow Knight
i could easily list off a dozen different bosses from Hollow Knight, but if I had to choose just one, it would be this one just due to how hard it is.
Ultra Necrozma - Pokemon Ultra Sun & Ultra Moon
or as I like to refer to it, Nuzlocke-Killer Necrozma. its hard to find anyone who did not immediately get their whole team OHKO's on their first try at this boss
Agent 2 / Commander Tartar - Splatoon 2: Octo Expansion
the Octo Expansion is the best thing to come out of Splatoon so far, and my god the final boss hits you like a truck. this one goes down as one of my most memorable boss fights ever.
You enter in a room full of mutilated corps and then you see something "kinda" resembling a human begging for help and talking a bout a beast(!?). Than comes Ludwig, at first a mindless beast, but soon it becomes a master swordman that uses the lengedary Moonlight Blade! Damn! Ludwig was especial as fuck! Lots of respect for Baldur in GoW, Friede on DS 3, lady Maria BB, Horizon Stormbringer, Ganon in zelda BotW and other...
I agree, Ludwig took my breathe away. Havent felt anything like it from any other game. That mid fight transformation was EPIC.
That video you showed didnt do the fight any justice, the guy was obviously over-leveled and simply overpowered the boss. Go watch some videos of "first timers" fighting him.
Void Termina/Astral Birth Void from Kirby Star Allies. (I'll never shut up about the game at this rate). Probably my favorite boss fight ever.
The build up of the ultimate battle for Kirby, a god of destruction, a destroyer of worlds, the swapping sequences of shoot em up and platforming and the origin and ties to the lore of the series, the final reward for the hardest difficulty of the true arena.
All backed up with a beautiful symphony, fittingly titled for Kirby's finest moment.
Is tough, is emotional and just an absolute spectacle to play through.