9. Mr. Nimbus
He's Mr. Nimbus. He controls the police!
When it comes to selling a villain on Rick and Morty, there's no surer way than to present Rick as actually flustered by an antagonist. Rick refers to Nimbus throughout "Mort Dinner Rick Andre" as his nemesis, which prompts the rest of the Smith family to react in surprise: Rick has a nemesis? But by acknowledging Nimbus as his nemesis, even while disparaging his rival's abilities a la Aquaman (or more accurately, Marvel Comic's Namor), Rick sells the idea that Nimbus actually is a threat.
Plus, he's frankly the only character on the show to call Rick Sanchez "Richard." Not even Bird Person has managed to get away with that.
Mr. Nimbus would be higher up on the list, were it not for the fact that he and Rick have an uneasy reliance and a begrudging respect for one another.