I like Shake King because he's got a lot of parallels to Wario, and indeed makes a great rival, but he's not a rival in the same way Syrup is to Wario. Syrup and Wario are both equally as crazy about the money and are sometimes villians too, but they aren't really intrinsically evil. When you shakedown with the Shake King, he's got all of your moves, he can shake the ground, shake you, throw you, dash at you, and you can do all those things back to him. It's such a cool and memorable fight because it has a unique feeling of Wario fighting a worse version of himself