When taking all aspects into account, Ornstein and Smough.They are the most fun to fight, have the most interesting lore, and a solid memorable theme. They are more than just a reflex text and require good positioning to beat.
Nameless king in dark souls 3 is my absolute favourite. Not only is the entire area well hidden but he's also the most difficult boss of the entire base game. Took me multiple hours to beat him. While the first phase against the dragon itself is kind of frustrating due to the bad camera, the second phase against the nameless king is soo intense and much fun once you learn all the patterns
Definitely, slayer Ornstein and executioner Smough from Dark Souls. Very balanced attacks from both bosses. At first, one would think that their duo is unbeatable but learning their timings and moves then one can beat them. Overall nice addition to the series.
