Grand Theft Auto III/Awesome


 * 'The Exchange'. Last mission where you FINALLY get to kill, and with a rocket launcher no less. Made even more satisfying by the difficulty: you start with no guns and there's no checkpoints along the way. Not to mention the news report cutscene that ends it.
 * Flying the Dodo (a plane with crippled flight mechanics) succesfully. Around the city. And even to "Ghost Town", the (normally unreachable) part of the city where the intro takes place.