The longest one is Promised Consort Radahn, 8 days of trying and learning. Amazing feeling afterwards, still like this boss (gameplay-wise) even though i understand all the hate around it.
Malenia was tough as well, i remember i gave up on her in the first playthrough and summoned people. On replay I killed her with two halberds doing insane amount of damage from jump attack and it felt underwhelming. Then i replayed the game again and defeated her using a twinblade, was very satisfying properly learning the moveset.
In souls series Friede was the toughest one, spend 5 nights on her (5 nights at Friede's lol). And I consider this fight the best in the series overall, especially story-wise. The buildup is an absolute masterpiece: how you meet her as a nice friendly NPC, then discover her dark secrets, then fight with Vilhelm, find Ariandel—and the fight itself as the climax of the story. Absolute cinema