I believe the most overrated character in the entire series has to be Sasuke. From the moment this series start he was the child prodigy with the bloodline limit that was oh-so powerful. I really couldn't stand his emo 'no-one can understand my pain' crap. Most of the reasoning behind his decisions was at best that of a sulking child. I think the moment for me that was the final straw for Sasuke's behaviour was in part one, when upon seeing the power behind Naruto's Rasengan he had a tantrum. The worst of it is that he's got twenty times worse since part one. To be honest I don't really care how he feels or if he can be redeemed. I just want Naruto to reach his goal of saving him and that is purely because I love Naruto so much as a character and he's invested so much time on this worthless creep. -Cough- Um...end of rant?
The most underrated character for me would have to be Hinata. She's never really been given the chance to prove herself yet she's always in the background fighting away. She's a Hyuga and should not be taken lightly. Yet all we've seen of her so far is her fainting in front of Naruto God knows how many times. I think she has real potential if Kishimoto would take the time to actually develop his female characters. Maybe spending less time on Sasuke would help. Heh.