19. Juggernaut

Cain Marko is a relatively straight-forward villain, but an iconic and important one nonetheless. Though his recent history has been filled with a failed attempt to be heroic, most IGN readers - and the editors at IGN for that matter - see him as a classic X-Men foe.

Marko served in the Korean War alongside his step-brother, Charles Xavier. When the two ran across an enchanted cave, Marko managed to acquire the Crimson Gem of Cyttorak and became the Juggernaut. Years later, Marko would arrive at the doors of Xavier's mansion, seeking revenge. Over the years the Juggernaut's history was developed to portray the villain in a more sympathetic light. He wasn't simply evil - he had a dark childhood and hated his step-brother for the advantages he perceived Xavier had.

Throughout his career, Juggernaut has clashed with the X-Men, Hulk, Thor and, in some ways most notably, Spider-Man in a classic two-part arc. His motivations might be a bit simple, but it's hard to argue against the character's popularity, design and impact on the X-Men and their founder.