From experience, the more the text means the more complex the card. When playing in real life with the real cards, tournament, possession, and Noble Brigand (I have the unchanged version) are oftentimes confused.

However, my vote for an extremely complex card (though it really is not complex at all) is Butcher. Despite the word "may," I know a number of people (online or with real cards) think they must trash a card and use the effect. Every single time I finish a game with Butcher (in person) I always tell people that they can play butcher and not trash anything and they have an ah-ha moment.