Basketball. If grip is your main concern, simply play basketball. I remember during fitness tests at my first university after I squeezed grip measuring device outside its scale, the coach remarked that I surely must be playing basketball, he was seeing it all the time.
If you're just shooting hoops, it won't help. If you're actually playing the team sport with aggressive opponents (in pickup or with refs who "let 'em play"), the ball will get ripped out of your hands every time you pick it up, and then your grip strength will improve.
