Nameless King for me, from just about every perspective. I immediately loved the visuals, from the storm to how badass that dude looked riding a dragon and throwing lightening bolts at me. I also really like the lore, how he is almost certainly the firstborn son of Gwyn. And while the camera might be a littler finicky in the first phase, I feel like they did everything they could to make it manageable and still even have a boss of that size and maneuverability. It's a huge arena with very few obstacles. The second phase combat was really challenging and rewarding as well.
The Dancer actually ended up being really memorable for me too. Not only did I love her fluid animation and how the chapel around the fight progressively became lit on fire with her blade strokes, but it was even more tense for me specifically because I triggered the fight *immediately* after defeating Yhorm. I only had half my flasks to start and wasn't wearing my ideal gear. I ended up beating her my first try with no Estus left and only about 10% of my health remaining. It felt awesome.
While it certainly wouldn't make my top 5 overall of anything, I would also like to throw an honorable mention to the Ancient Wyvern. I think that was without a doubt the best "puzzle" boss that this series has had. It wasn't all that difficult but it looked awesome.
I really loved the bosses as a whole in this game and I'm really excited for more in the DLC.