I'm not talking about favourite boss fights. I'm talking about boss fights that, for one reason or another, were remarkable, unusual, or unexpected, or something along those lines.
The first one might be a bit boring, but it's still somewhat memorable to me; Sister Friede from Dark Souls 3. The reason for all 3 phases of the fight standing out to me was because it felt so much like it came straight out of Bloodborne, with phase 3 especially reminding me a lot of Gherman. On top of that I swear to god that, while it's still unconfirmed, Ariandel has possession of the Lordvessel from the first game and even uses it as a weapon of limitless fire against you in the second phase of the fight. Like I said, while it's still unconfirmed, the resemblance to the Lordvessel is too uncanny, even though I'm not really one for lore speculation.
The second one I can remember right now would be Mr. Freeze, specifically from Arkham City. The reason his boss fight stood out to me was simply because of how he could adapt to your techniques, meaning you had to use more or less all means at your disposal in order to win, and I don't know if any game has implemented such a system of adaptability aside from Shadow of War, where higher-ranking Uruks could learn to counter your attacks if you used the same ones constantly.
Finally…
Out of all the boss fights I can remember right now, Toru Hirose is the one that stands up to me right now, the primary reason being for the drastic change in gameplay, not just in this one game, but the entire series. Every enemy you could ever face throughout the whole Yakuza franchise is aggressive, fairly quick, and with at least some level of efficiency in hand-to-hand combat. Hirose, on the other hand, is the complete polar opposite. He is slow and methodical, rarely making the first punch, staring you down before suddenly lunging forwards with his unblockable knife. The incredible music is also very much turned upside-down; the vast majority of notable fight music in the Yakuza series is either blood-pumping or an epic orchestral mix perfect for a final boss, or maybe both at once. This music that play during this fight, however, would not be out of place in a horror or disaster movie, and fits perfectly with how he is eventually brought into being allegedly one of the strongest and deadliest men in Japan, to the extent of making the leader of a brutal and merciless Chinese triad sweat with pure fear.
He's also modelled after and voiced by Takeshi Kitano.
What about yours?