The unique part is its main character. He is not just a typical assassin. He has his own code of ethics that sets him apart from others in his line of work.
A unique setting can make it stand out. For example, if the assassin operates in a world with a strange magic system or a very different historical period.
It's hard to say without reading it. However, the main character might be a complex figure. They could be someone who was forced into the assassin's life due to certain circumstances, like a tragic past event that led them down this path. Or they could be a character who enjoys the thrill and power that comes with being an assassin.
The main character in this 'world's finest assassin novel' could be a female assassin, which would be a refreshing take. She might use her femininity as a weapon, being underestimated by her targets and then using that to her advantage. Or it could be an older assassin who has seen it all and is nearing the end of his career but has one last big mission to complete. There are so many possibilities without more information about the novel.
It depends on various factors. Some people are really into assassin novels because they offer a different kind of hero or anti - hero. If 'worlds finest assassin novel' has a great narrative that explores the character's journey, motives, and the world they operate in, it could gain a following. Also, if it manages to stand out from the crowd of other assassin - themed books with its own unique style, it has a good chance of being popular.
The plot might be one of the reasons. It could have a complex and engaging story line that sets it apart. Maybe the characters are well - developed, with the assassin having unique traits and a fascinating backstory.