Because he's good at being evil. He has his flaws, like most villains, but he's an incredibly intelligent and competent antagonist who actually succeeded in his plans, even if only for a brief period of time. Not to mention the sheer scope of his plans makes you appreciate his genius. They also made him incredibly powerful, something anyone can appreciate. Also, because he's evil, we don't have to debate about how to feel about him because we all agree he's the worst, yet we can appreciate him as a villain.
His personality 
His powers
His Genius level of intelligence 
His cruelty
