I would say a 12 or 13. It was way better than the original. The only major letdown was with some of the new characters. I liked the characters themselves, but most of their special techniques were more like taking a break and letting their squad members attack. I hope in the third game (hopefully I can find a copy later today) each character has totally unique specials. All in all, though, it was still a good game.
I'd give it a 11. It wasn't as addictive as the first game, but it did have a larger selection of characters and stages and a MUCH longer story mode. I hate when the characters specials are Ultimate Mode or one of their teammates specials. However, I love the idea of Pest characters and the minigames. Also, the graphics could have been updated from the Japanese version's, but they weren't (or not noticeably).
Out of 15...hmm...I would give Naruto Ulitmate Ninja 2...a...12 out of 15...there's still a bit more to be desired...we could've had a bit more...=-_-=