Bee said most powerful genjutsu user. He is a ka user and thats how i translate it. Also you cannot create ka, its an ms technique that you get. I do however believe that his ability to use ka is more effective that anybody else (danzo). Kinda like how itachi has tsukuyomi and his is the most potent, sasuke's isnt exactly up to itachi's level.It's already clear who was the best Genjutsu user - Shisui Uchiha. Excepting Shisui, Itachi is the best.
Bee said he was the strongest Genjutsu user and not he was able to use the strongest Genjutsu. I think his strength in Genjutsus has enabled him to create the most powerful Genjutsu - Kotoamatsukami.
Anyway as i said even if bee said skillful, believe itachi has surpassed him