What you should pay attention to is the monotony of the game. When you get to an advanced level in any game, stop playing it. Change the game, and preferably the type, from FPS to RTS, from RTS to RPG, from RPG to adventure games. Move from adventure games to JRPGs, and then you have simulators, economic games, tower defense and PC board games, etc. They all improve the brain in some way.
If the games have single and multiplayer mode - beat the campaign on the hardest difficulty first. When you do that, start playing multiplayer - try to defeat better and better opponents, not using ready-made tactics created by game masters. Challenge yourself.