Have to go old school MP and say Unreal Tournament (original, but 2003/2004 as well) and Quake 3.
Those games were fast. Ridiculously fast. Even more so in UT when someone increases the Game Speed setting for fun. 16 player Instagib deathmatch at 200% Game Speed seperates men from boys, women from girls. Even without mods/mutators the normal games themselves require great aiming, reflexes, spatial awareness, memory, time-keeping and mobility.
Always fun getting someone considered a good Halo or CoD player to try one of those games against reasonable bots. They get slaughtered near every time.