Konohamaru is probably the worst. Kidomaru's 2nd state is also pretty terrible. Regular Kidomaru is strong, but his*2nd level curse mark form uses his bow too much (there are no combo attacks to use). That said, I am good with every character, I just have to play differently when using different characters. Most characters I use throws and jump-dash combos and jutsu. Some of the weaker characters, however, I tend to rely more on Ultimate Jutsu.
Yeah I have to say Konohamaru is the hardest character to win with.
*This is mainly because he is an "odd" character with only 1 land combo ( A combo move) His guard break isn't a good guardbreaker ( unless you charge it up ) But in the long run, I would agree with what BLACKCHIDORI101 said.