I think it's fair to say that Painkiller has similarities with all 3 of those games. In SP, it's reminiscent of classic Doom, which Serious Sam harkens back to in terms of the swarms of enemies and exaggerated weaponry. In MP, however, Painkiller plays most like Quake (QuakeWorld in particular). There is an emphasis on item control (getting armor, weapons, etc.) and movement (bunnyhopping and rocket jumping). But when taking into account the physics of the game, I think PK is most comparable to QuakeWorld; it's just insanely fast. The player can change their direction in mid air while bunnyhopping, weapon switches are quick, and projectile weapons are really fast. PK takes most of the aspects of QW (and abandoned in later iterations of Quake) and puts them into a more modern package. But I think anyone that enjoys Doom, Quake, Serious Sam, Duke Nukem or any old school FPS can draw similarities.