Shusui being more talented than Itachi? I am not sure.
Sure he was strong but according to the databook, Itachi chosen as the spy because of his talent. Danzo didnt even trust Shisui, and the clan decided to have Itachi as the main pipeline... this should kinda hint that Itachi was the best at doing the job.
Obito freaking sky rocketed without a doubt, and yeah he may be stronger than Itachi.... but it's also A LOT due to those damn Senju cellls, it's not only because of his talent and skills,
Besides... given that Izanami is a way to stop Izanagi user... we could say that Obito is vulnerable to it.... Itachi has what it takes to take Obito on, Obito did say it himself after all
Had Itachi known his secret, Obito would be dead.
Meaning if Itachi knows everything about Obito like Obito knew everything about Itachi,
Itachi is the declared winner. So with that said, Itachi is stronger than Obito.
Obito is only the winner when specific circumstances are met, like Itachi not knowing a thing about him.