Mega Man X. The bosses are tough, the weapons you get by beating them kick ass, you can do some stylish stuff (like jumping off a wall to avoid Spark Mandrill's wall-climbing sparks, then turning around in mid-air to grab the same wall, and then jumping off the wall again as soon as you touch it and over Spark Mandrill's head as he tries to knock you off the wall with a dash punch, then retaliating by firing a charge shot at his back), the music rocks (especially Sigma stage 2) and the final boss fight is really hard (if you don't do it the cheap way by standing on the edge of one of his hands).