They elected kings but the obvious strongest contender was often the son or heir of the previous king, leading to rival dynasties of which the strongest would be the Vasa, the Electors/Kings of Saxony (Wettin) and the House of Poniatowski. By the end of the 18th century the question of succession was basically alternating between these two rival dynasties, whilst the kingdom itself was being torn apart - that is IIRC, which is always a good question at the moment, it being a while since I read about this.
One note in addition is that of course the LAST King of Poland legally speaking was the Russian Tsar who was created King of Poland under the Congress of Vienna, and who held it as a subsidiary title until the kingdom was dissolved in, I think 1830 as a result of widespread rebellion - I think this makes Nikolai I the last legal King.
Additional to this note, is that the last CLAIMANT was one of the leaders of this revolution, and who lived in exile in Paris afterwards addressed formally by everyone (at least in the West of Europe) as de jure King of Poland.
Best Regards
Grey Wolf