OMG! Grand Theft Auto 3. For a mature kid or an immature adult it is the best game ever made in my opinion!. Where else can you carjack soccermoms, steal a LAW from the reserve base and shoot down a police helicopter, assault pedestrians with a basball bat, shoot people in the head with an m-16 and watch the fountian of blood flow, screw hookers for extra health points, climb a belltower on a universtity campus and let fly with a m98 sniper rifle, steal a freightliner and jump it like Evil Kneivel, participate in organized crime, steal an infantry fighting vehicle and clear a road like the guy did in San Diego, get "3 wheel motion" in a lowrider, take meth and see the game in slow mo while your haymaker punch sends people 23 feet into the air? HUH HUH no this is the absolute coolest game ever made. Of course I do not advocate any of these activities but to see a video game in which your charcter gives cops the finger while jay-walking is just super cool.
my .02