From a playtime POV it would be Left 4 Dead.
Now I'm currently playing Helldvivers 2 far too much.
Over the decades it was probably anything that ran on the Quake 3 engine.
I feel old now 😉

Used to play Counterstrike beta when I was young, played a lot and was pretty good at it. Nowadays I play teamfortress 2 but much more casually.

Tbh i really liked the gunplay in Black Ops 2, as simple as it was

Thank you for the game :D