The greatest video "games" ever made are:

[Rai]

Serious Sam (with The Second Encounter included) is probably my favorite game ever. It's easily the best example I can find of perfected simplicity. Painkiller comes close, but it loses steam partway through, while Serious Sam keeps itself going through regularly introducing bizarre situations in which to fight (the bouncy floor fight and the fight that takes place mostly in the dark come to mind)