TimeSplitters 2
To this day, this game remains my favourite local-multiplayer FPS experience. It had both great maps and great weapons. My roomates and I in university spent far more hours than we should have on this game; it just simply never grew tiresome. The reason it places so low on my list is not due to its multiplayer merits but due to its comparatively lackluster singleplayer.
I really, really hate those damn monkeys, though.