Voted for Rudy. I don't think Dr. Mario 64 really helps him when he's so weak in that game wanting to just cure his cold, but it's nice he's in that regardless. I like his design, the fact he tricks Wario, is the only boss that can kill him, and started his hilarious fear/hatred of clowns.
Captain Syrup and Shake King are also great choices. There's really no point in talking about Syrup, but Shake King had a pretty great boss battle and powers, and it was great to see Wario actually go up against more of a brute character as the main antagonist (Even if he does have magic).