After fighting through every other character in Street Fighter II you have to take down the final boss, M. Bison, who naturally is also the most infuriatingly difficult of the lot. His Psycho Crusher attack in particular caused me to lose to him a lot, and I generally find that playing the defensive is a much easier way to win than going outright on the offensive. Bison can reverse the vast majority of your attacks if you get even slightly overzealous, but take a cautious approach and you'll find yourself slowly whittling his health down. Keep it up and you have a good chance of winning, bringing an end to one of the greatest beat 'em ups in the history of video games. Beat Bison (or Vega as he was originally called in the Japanese version) and consider yourself a master of the classic brawler.