Blazblue Calamity Trigger
Hoo boy, I also have to say Unlimited Ragna, but the Calamity Trigger variant, who stands as the Score Attack final boss. Now, why this is so hard? Well not only does the AI get a kick out of standing above "Hell" difficulty, the highest one you can choose for the other modes, being this mode unique in that sense, but you are also facing the strongest rushdown character in the game, with the craziest unlimited of them all. Sure, they all get stronger and shit, but Ragna can recover his entire health bar with just a few hits while also hitting like a god damn truck, like seriously his damage output is crazy, not to mention his movement speed is way higher, AND THEN THE HP. The only way to actually win is to think quick, move around and not even try to guard unless necessary, because the HP steal happens anyways, instead, you need to be cheap, don't even try for most combos cuz when he gets up, he will probably abuse the fact you are recovering, and a single hit means you are getting combo'd to death, or losing a shit ton of health.
I would honestly consider CT Unlimited Ragna the strongest he has ever been, because I fought him in different modes in CSE, CP and even CF, but man... nothing prepares you, it's busted.
Dark Souls (Remastered)
Ornstein and Smough, do I really have to say more? Many people say Manus or like Artorias, but Artorias had me figure out his patterns after like a few attempts and then that was it, just control my usual "RUSH THIS MOTHERFUCKER, GO WILD" style of playing when it comes to action, which actually helped then with Manus, cuz I was super aggressive and none of the moves it was using was making me stagger anyways, so I would just use my Black Knight Sword, armor and maybe the shield from time to time and chug Estus. No biggie, was my bitch. The way to Manus was harder than Manus. Anyways, the point is this duo was my bane, why? cause of the previosu reasons, I'm too aggressive for my own good, and since there are two bosses to consider at once, I really had to keep my guard up while landing hits and play it safe, which took me so many tries, oh so many tries cuz even after I had memorized the patterns, I was still making some risky plays and gettined nae nae'd. Eventually I won though, so I consider it hard because I finally met my match in-game, not even the final boss gave me as much issue, again another first try situation. But to be fair, I was already experienced with DS3 and Bloodborne previously, so that may have helped. Speaking of.
Bloodborne
Gehrman, my lord, Gerhman. Fuck the hidden boss right after, THIS is the true challenge. His speed, his attacks, his moveset, the music. Similar to Gwyn, you had to parry to make it happen, or hit hard enough for it to not be necessary. As expected, in my playthough I was using Ludwig's Holy Blade as my weapon and never really figured out parrying. With this in mind, I had to learn his moves and be able to predict what he would do based on where I was standing and what will follow after each start-up animation, which iirc was really quick, what always stuck with me was his pure speed. Anyways, fucker had me on my toes, had to keep dodging and trying to parry the easier attacks, learning as I went along and also making sure not to die too quicky and save up on health items, like "Can only heal if I reach X point". It's a real climactic fight, forcing you to use all you've learned so far, and even then I was forced to learn more to win. Hard as balls but so good.
Dark Souls 3
Midir, fuck this guy, fuck this dragon. I was only able to win with my Guts Cosplay Build, pure STR goodness. An endurance fight which had me anxious as fuck the entire time through. Harder than Gael by a mile, but not as cool. There isn't much to add about him besides... massive damage, massive reach, a gigantic HP pool and pleeeeenty of one-kill hits. I was so happy when my "Dragonslayer" lived up to its name <3 <3