Akuma is probably one of the most recognized bosses in fighting game history. His introduction in SSF2T was mysterious and left an impact that carried into Street Fighter history. At first glance he was another Ryu/Ken copy, but with the ground-breaking introduction of super moves, Akuma was given the coolest super ever; the Raging Demon.
Essentially a command grapple, he would rush up to his opponent, the screen would black out for a few seconds and when you reappeared his enemy would be lying on the floor beaten. It couldn't be blocked and there was a wake-up trap for people who actually knew how to use it against the less seasoned players. If it killed you his Kanji character would appear in his sunburst flashy finish. For balancing issues Akuma is usually given a paper thin defense since his introduction, making him a glass cannon. Give him to the AI though and you'd have quite a fight on your hands.