Hmm ... thats a hard one. The one fight he always gets owned and would be embarrissing: Sakura-chan. I mean common, a dim wit half ass ninja with no skill owns him every time. You can't really say its Tsunade because of her insane strength IMHO. But ... for all around hardest fight for him to take its a tie: Nagi(forced him to use Kyuubi like it was said earlier) and Kakashi(1000 years of pain).