For me its CS 1.6
With all its crazy wallhacks, kinda hialrious
To be honest, "hacking" in all games such as CS, MW, BF and so on where multiplayer and teamwork is the main thing about the game being fun is worthy of a ban for life time from all thoes games.. It's one thing to have some fun when playing against real life friends just to piss them off, but taking it as far as public VAC secured servers and so on is just selfish imo.
Nevertheless, finding spots to jump and exploit the maps is another thing and I've had a lot of fun stacking people in towers just to abuse the map it self in ways never imagined by the creator of it.