Naruto Clash of Ninja Revolution 3! I'm going to keep mentioning it until it becomes a fad. It's awesome. The Wi-Fi is great, it's a lot of fun, and there are 40 mostly unique characters. It was developed by the same people who are developing Tatsunoko vs. Capcom. I'm starting to really like them!
Little King's Story and Final Fantasy Fables: Chocobo's Dungeon too.