I think one should exclude Naruto and Sasuke because they are obviously the most developed chars.
If not, then the plot wouldn't be great as it is now.
I pick Hinata, not only because she is my favorite character, but she has had a decent development.
She had a not so many panels in the manga, a secondary character, but Kishi has been able to showcase her emotions, past life, background and development.
Development clearly manifests in a considerable change in power, attitude and skill, which Hinata has shown.
She was known as a weak Hyuuga, but now she is living to his father's standard, or at least she is getting closer to that.
Other notable developments:
Sakura
Lee(has fewer panels to show though)
Ino(man, she's pretty useful in war)