Slave Knight Gael
Gael's truly the greatest (real) finale that Dark Souls as a franchise could have asked for.
Not only does he absolutely fucking nail his presentation; The move-set, the lore, the desolate arena, the roll catches, the weaponry, the phases, the dialogue, the cutscene, and the score are all top-notch. Gael is, in my opinion, unironically the epitome of FromSoftware boss design and I genuinely believe nothing they ever do can top him.
He's certainly not the hardest boss or anything, but to say he's one of the things I look forward to the most in Dark Souls III's endgame would be an egregious understatement. I think it's safe to state that nary an expense was spared for his design.
Gael's a boss that I (admittedly unknowingly) trivialised for myself my first couple of runs by virtue of using my favourite weapon, the Hollowslayer Greatsword, which deals extra damage to him in certain phases - so while I did always like him, I never really got the opportunity to 'bond' with him per se because I managed to dispatch him fairly quickly.
The sheer grandeur of the arena, the music, and the man himself all consolidated together, serve as the perfect sendoff for the trilogy. Gael in particular stands out as one of the greatest in a game already known for having phenomenal bosses - easily the greatest I've encountered personally, across any game I've played, but I digress.
The sheer ferocity, the unhinged aggression, the 'fuck-you' attacks what with the teleport, repeating crossbow (clearly an allusion to Guts from Berserk) all make for a nail-bitingly tough but satisfying encounter. Despite all his other flaws, Gael, like most bosses in Dark Souls 3, feels fair.
The lore implications with his presence in the Painted World are also great. I'm a sucker for bosses that have good lore and actual plot relevance and the way Dark Souls 3 pulled this off so consistently for so many bosses, and Gael still manages to hold his own.
The fight itself is absolutely anime as fuck. I love how it manages to be flashy what with the lightning strikes and his cloak's crimson bellowing but not to the point where it hampers your experience and/or visibility (like, say Laxasia or the final boss in Lies of P).
The subtle fake-outs, the clear-cut tells, the leaps, the plunges, the magic, the roll catches - Gael has quickly rose to become one of my favourite Dark Souls 3 bosses and I had an absolute blast beating him most recently with the Dragonslayer Greataxe.