He, for a very long time, was my perfect villain.
No sob story, no justifiable motives, just a cold-hearted money-obsessed psychopath who'd double cross anyone as long as there's a profit in it. He's smart enough to play both Kingpin and Osborn like fiddles, he relies solely on his skills, tech and intelligence. He's charismatic beyond belief.
Megabyte is the only other villain that left as much of an impression on me as this guy did.