Among other criteria, I'd have to consider what was the most powerful business model in the world for online commerce and take that category killer, in a specific and focused niche. Perhaps:
Poker.com
That's why I don't consider Music.com or Sex.com to be top choices anymore. Both industries have been absolutely destroyed by "free content" and "cheaper content" models than were prevalent even as recently as ten years ago. Talk to anyone in either industry and they'll tell you that the industry has been in a gradual, sustained decline.