Have to agree, anything large and responsive. People tend to be impressed by something that isn't the traditional fish shape too - I mean an oscar is cool, and has lots of personality, but everyone has seen one. Non fishkeepers are usually wowed even by my little 9" BGK - nevermind an adult CK or something. People tend to love my large plecos too. a very large fire eel is tough to beat for the "holy crap" factor from fishkeepers and non fishkeepers alike.