Halo and COD, pfft, whatever.
you want the A-list of first person shooters? Revisit the classics like Battlezone
or the much later Wolfenstein 3D
Modern first-person shooters are essentially carbon copies of each other anyway. Only this time, instead of a Bazooka firing on Japanese Tank, my MC gets to take a Wraith down with his plasma launcher. Ech, how about some in-game ingenuity, or puzzles, or a game where increasing difficulty does more than just make your character's health bar more fragile...