The original Serious Sam from 2001 is the very first first-person shooter I ever played.
Serious sam is a representative of spin-off shooters where the developers worked not so much on the plot as on the gameplay. The essence of the game is only to shoot a bunch of enemies with different guns, of which there are enough in the game, the original game looks very outdated, but the developers have released an HD remaster that still looks nice. The first Serious Sam looks worse than other parts (if you do not compare the remaster with the second part), but it is the first SS (TSE/TFE) that is the most dynamic and fun parts.