Killzone 2.
After Killzone 2 and 3, as well as the reboot of MOH I just can't enjoy a FPS as much if it doesnt have leaning in, out and over cover to shoot. As much as I enjoyed Black Ops (the last COD I played) it just feels silly having to either physically move around a corner or stand up from behind cover to shoot.
Some people hate FPS games with cover shooting mechanics but I find it hard to go back now that alot of current gen games have implemented it so well.
Edit: I suppose it depends on what the game wants to achieve and what tone it wants to set wether or not a cover system or a basic lean function would work in a FPS.