The key to me is that great casual cards are fun, and this also generally means that they are among other things playable, promote game interaction, produce a strong effect, encourage creative deck building, and then to some extent I also look at things like whether it was played in tournaments, its use in multiplayer or other more casual formats and its flavor. However, I don't consider casual to be the opposite of tournament, but rather to be its own category. With this in mind, I believe I shall nominate the previously mentioned Squirrel Wrangler. The guy is strong, encourages some decks to grow up around him, and has the appropriate flavor.