Casimir III is an obvious choice - the only Polish monarch called "the Great" in Polish historiography. He changed Poland from weak, backward country into a strong, well organized state, with good economy and modern infrastructure, a significant player in European politics. As Poles say "Zastał Polskę drewnianą, a zostawił murowaną" (When he ascended Poland was wooden, when he left us, it was from stone - it looses in translation).
Another of my favourites is first historical Polish monarch, Mieszko I. He introduced Poland to European world and culture, won first historical Polish battle (Battle of Cedynia) and conquered Silesia and Little Poland, defeating Bohemians rather by political than military manouvers.
I agree with Jonathan, that Stanisław August is seriously underestimated: he had to play against three powerful players, united against Poland, and his cards were very weak, and not by his fault. He achieved a lot, considering circumstances.