I own them all thus far, and finished most of them. Having given it some serious thought, and I hope others agree, I am going to have to go with Black Flag. I remember being stunned by the visuals. Especially at sea, boat getting battered, waves riding high...it felt like chaos, but in a good way. Loved the story too..man, you had to put in some serious serious hours to get EVERYTHING in black flag.
I would totally appreciate a Black Flag remake in Unreal Engine 5. At £30-£35 quid, I'd put my first ever pre-order down rapidly.
And also purchase extra pants..I'd be shitting myself with excitement.