It really depends on what you want to improve. Games that improve motor skills are defintely the 1st person shooter games because they involve the user to focus; sight, touch, hearing, all at once. When i say touch, I mean the ability to click the mouse or a button on the d-pad. Ill get into even more detail below:
BlackOps: This game requires the use of the analog and you need to be good at it. And when im saying good, I'm saying you need to be able to move your cross hair into the direction of an enemy as soon as you see/hear and move your fingers holding the controller a fixed distance to engage. This isnt the only thing. When you throw grenades, even though there is a fixed distance from all angles, you can control the distance by choosing which angle to throw at. This may seem easy at first but once you're in a game in real time, time is not on your side and this requires very quick thinking in a short amount of time. Things improved : Situation assessment, reaction time, hand eye coordination, calculation.
Racing games: These type of games require slow calculating precise movements in comparison to blackops. Calculation comes from knowing when to make turns, downshifting, drifting; slow precise movements are needed during turns at high speeds, making the turns, etc. Things improved: Slow, precise, calculating movements.