I personally love doing Cork 5's. IMO they are so damn fun to just carve off and float.
Cork 7's are something you should do on tramp first. Do you have them perfect on tramp? If yes, then just throw it on snow. If no, practice them on tramp. Same with cork 5.
So, I think if you've never done any of them, then I'd say Cork 3. Just carve off the lip and drive with your outside hip around the other hip.