Void is one of my favorite cards. One of my favorite scenes in the whole movie is when all of the Sakura Cards leave her and she states, "Doushite (Why)?!" I started to cry at that one, and then when Sakura is crying over (can't tell spoilers) Hope states, "Daijobu (It's alright)." My eyes were filled with tears.
But another card, that I wouldn't say is my favorite, but it entrigues me, is the Power Card! When you see what power can do, you expect it to be like a sumo wrestler or Hulk Hogan, but you see its form and it totally throws you for a loop! I won't state what it looks like in case you haven't seen the episode, but once you see it, you'll be surprised.
But my absolute favorite is the fly. Especially in the Sakura Card Arc! Again can't tell why due to spoilers.