Actually i disagree. Im certain shisui was a genjutsu master since he was like itachi's older brother/sensei therefore i wouldnt even be surprised if its shisui who thought itachi genjutsu, but you see itachi has surpassed his sensei.
Ao had fought him and its to this that he knew about KA. Yet when the time came both him and shikaku another leaf ninja who wouldve knew shisui, mentioned itachi as the only possible person to be able to control others from a long distance.
Bee what he said can be translated in two different was and the way that it seems for logical to me at least is that shisui was the strongest genjutsu user of the uchiha, meaning he had the strongest genjutsu and he was the user of that genjutsu.
Secondly no, i do not believe that ka is the strongest genjutsu for many reasons.
Firstly ive noticed a pattern in relation to uchiha techniques, the latter a technique is revealed the stronger it is to its predecessors. So for example we heard of tsukuyomi first and thought it was op, then amaterasu came which was even more op, then susano came which the manga has made it very clear that out of the 3 that is the ultimate technique.
Note that zetsu called tsukuyomi the strongest genjutsu. After finding out ka, that then gained the title of strongest genjutsu, that title was short lived when we were introduced to izanagi and now that title has been taken by izanami which has been called the ultimate technique which decides your opponents destiny. Its very clear that ka, along with izanami and izanami belong in their own category, but imo its between izanagi and izamami. KA was broken and alot of people still dont understand. It takes control over the person without them knowing. Just like izanagi and izanami it doesnt take into consideration your 5 senses. But ka works by taking control of your opponent without them knowing. Once they know what is going on the technique has been broken. Once the techniqiue was found out by Ao and he informed the summit, mifune came back his senses. Therefore in order to break it you must know what is going on.
Izanagi is more of a defensive move for obvious reasons. Izanami is an offensive move it it affects your opponent and as i said since that is the latest technique to be revealed its most likely slightly greater than izanagi