Admittedly, his shock power is kinda awesome.

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.

All of the above. Well-written villains are distinguished precisely by the combination of various personal qualities. If Palpatine had been cruel but dumb or very smart but at the same time uncharismatic or wimp, he would not have become such a iconic character. He is charming yet utterly deceitful, he is genius, but he makes mistakes, he is monstrously powerful, but he pretends to be a feeble old man and this makes him a magnificent villain.

I answer that his personality, since the other options form it.