the new DOOM is hardcore as fuck, highly recommended. Painkiller Black Edition is also great.
Of course, I can easily recommend some classic shooters: (assuming you havent played any of these...)
Doom 1,2,Final + Master levels+64 EX, Quake+Mission packs, Blood, Duke3D, Outlaws, Star Wars: Dark Forces, Unreal Gold, Descent 1+2, Shadow Warrior (classic and modern versions)