Halo. It's not a genre defining game,
Then why do almost all FPS games ever since Halo have a recharging health system and allow you to carry only two weapons at once? Checkpoint system? Controller layout?
It hasn't re-defined the entire genre, but it sure made some huge steps.
and the first one sold 5m+
It did a little better then that, actually. But it had very restricted multiplayer capabilities...