Gazeta Wyborcza is the biggest-selling serious newspaper (not a tabloid)
This is true. Fakt and SuperExpres sell quite well - they are closer to the New York Post in style.
Dziennik is popular, and Polska has the same owners as the NY Times. As Szwed w Polsce says, Rzeczpospolita is conservative, though paradoxically the newpaper of choice for many of the communist-era elite.