Madara while he was alive was the second srrongest ninja alive, able to stand up to Hashirama, albeit he couldn't defeat him in combat. Shisui doesn't have a chance unless he used Kotoamatsukami. But Madara basically stomps Shisui 10/10 times. Shisui is the strongest Uchiha of his time not all time.

Was Itachi stronger than shisui?

Danzo said shisui was most powerful uchiha in the uchiha clan and itachi was his equal he just didnt surpass him + he was the most talented student in the academy + was selected instead of shisui because of his ability instead of shisui even though shisui has the same openmindedness as itachi.

Who can beat shisui?

4 CAN BEAT: MINATO NAMIKAZE

Minato was capable of taking down entire armies on his own, and he was more than capable of beating Shisui.