You are too narrow minded. Read entire dialog, understand it, then talk. That's how literature works. Bee said Shisui was the best. Then Itachi continued by saying why he was considered the best. The reason was obviously Kotoamatsukami. The technique Shisui could use every 10 years.Kishi's words>anything you say. He's said to be the strongest genjutsu user of the uchiha. What's so hard to understand about that?
Just an advice. We are all here to discuss the matter based on proves present in the Manga. Being Kishi's bi*ch is not considered discussing.