To be honest, any FPS game will improve your aim if you put enough time to practice in it. CS:GO has a bunch of workshop maps that are designed to improve your aim so that's an easy way to practice your aim if you want to put the time into it.
Any FPS game goes (just hope you don't bamboozle yourself with the wrong system from another game). All FPS games teach you one thing fundamentally, and that is to keep your aim at head level. The only other things you should take consideration into is map awareness and eye-hand coordination (i.e. your target is jumping or maybe appearing from an area above / below you). Of course, this is just basic knowledge. Anyone is welcome to provide a more in-depth explanation to mine or bring other details to mind while playing.
Make sure to practice the type of aim you want, like if you want headshot aim you shouldn't play many fpses like Rainbow 6, battlefield, cod, etc.
