The only game so far that has truly surprised me was Super Mario 3D Land. I had so much fun going through that game the first time through.
As for disappointments, Code of Princess and New Super Mario Bros 2 come to mind. Normally when I buy games I don't regret my purchases (even if it's a game I may not end up playing as much as others) but these two games are ones I really wish I hadn't bothered with. It's made me a much more cautious buyer when it comes to 3DS games.