Mh... hard to tell, there might be much more than what i remember, so i'm gonna mention among the latest ones:
FFXII Yazmat, fighting for 3 days... it happened
FFXII the zodiac age - the 5 judges at the end of the 100 stages mode, i aided my bro in fighting them by providing suggestions on strategy, he was underleveled but they were tremendous in any case. I loved how the battle was designed for using any possible feature and trick of the game, included spacing and distances and timing... it pretty much included all i wanted the game could ever implement all around the game (i think that modding could make wonders here)
Disgaea D2 Baal (the 400 million HP version for my own mental sanity), it was so cool, never saw such a great display of features and awesomeness all in one go in a single bossfight of a strategy JRPG game, i literally had to use everything here a fight for a whole night, trying to not make any serious mistake
Rabi Ribi - Irisu (hard mode) the game is incredible on its own, this battle is quite intense, and yet i still have to fight the power-up version of it which was available only after the ps4 latest platinum edition update
Rabi Ribi - No 0% item run, almost literally any bossfight... i really say what i mean: (well at least starting from hell difficulty or it would probably become much easier) every battle will make you sweat blood, you will never forget, it will stay impressed in your skin forever. In particuar i'm doing the Alternative Mode in which basically the avg level of bosses skyrockets, compared to standard mode... i can only say that such challenge is not supposed to be handled by humans, i'm trying my best and at least 2 years have passed since i started, but i'm quite close to its end. I can say with serenity that this will be hardest challenge i'll ever do for the rest of my life.