I'm torn.
Casimir III is out, for the sake of diversity.
The obvious choice would be Jan III Sobieski, he is basically a symbol of POLAND STRONK
Władysław II Jagiełło is a solid choice as well for the reasons I outlined before.
And there is Bolesław I Chrobry, the first crowned king of Poland, unlike the more warmongering kings abowe, he was more defensively focused and a very skilled diplomat.
But who I think I would choose Stefan Batory. He is just too badass - not only for deafeating Ivan the Terrible, but also how he handled the Danzig rebellion - the city of Danzig didn't want to accept him as a king, he decided to show it its place. This city was always a very delicate matter, wealthy, important for Poland and with with a net of connections with the most powerful European players, so the nobility wanted to settle things peacefully, and because of all the privileges they had they could just refuse to support war effort. Batory got pissed, took his 2000 loyal men, massacred five times more numerous forces of Danzig in the battle of Lubiszewo and besieged the city, until they reached an agreement.
He also modernized the army, founded the Vilnius University, created the jurisdictional system separate from the king, continiued the uniuque policy of tolerance for all religions and he was the king of Poland during its true golden age, while in the times of Jagiełło it wasn't there yet and when Sobieski got the crown the country was already going downhill.