Maybe not the worst, but he’s pretty close. We don’t get much of him in ESB so I can’t say he’s a good or bad villain. But in ROTJ he seems incredibly idiotic. Luke is about to kill Vader, but the Emperor interrupts him and essentially says, “Yes, kill your father so you can take his place at my side and be an evil, miserable POS for the rest of your life.”
He’s not clever AT ALL.

He taunts Luke saying that all his friends are gonna die, the rebel alliance will be destroyed. Luke is starting to feel anger and fear. Then the Emperor, instead of playing up the threat Luke’s friends face, he then says, “Yes, strike down, so you’re turn to the dark side will be complete.”
Obviously, the Emperor does not want to die. It’s pretty obvious that Luke should do the exact OPPOSITE of whatever the Emperor is saying.

I kind of wish he had more depth to him. Villains with vague motivations and unrevealed origins are fine, but at least make them visually interesting. Palpatine is just an old man in a black robe.

Why does he want to dominate the galaxy? He’s just a cackling, comic book villain, who’s just evil for the sake of evil. I like Star Wars, but he’s definitely not a very good villain. I did like his appearance in Strider though.