Pretty much all were very OP in thier own ways
As someone mentioned earlier it would have been interesting to see Oro and Sasori work together
I have a hard time picking though, Part of me wants to say Itachi and Kisame. but Pein alone was a 1 man army. I think the only reason kakuza/hidan/sasori/deidara lost was plot.
And about Kakuza and Hidan - Yes they got beat by, Kakashi,Shika,Choji,Ino,Naruto, and Yamato... but 90% of the NU would get owned by those 6 who already knew most of Kakuza/hidans powers, with Shikas brain. And you have to remember, they were stomping on Asuma,Shika,and the two other Jonin, and didnt even break a sweat when reinforcemts arrived.
Not saying they are the strogest. but give credit where credit is due
If I had to pick... Pein and Konan, mainly because of pein, I dont see any of the other teams figuring out his weakness before he kills them
best bets are itachi/kisame and sasori/deidara but i dont see it happening