There were alot of cool fights when they were chasing after Sasuke when he first left Konoha. . The whole incident with Itachi vs the jounins was really awesome as well. And in the beginning when Zabuza kicked the shit out of the gang with only a kunai in his mouth, that was a great fight to. But if we are talking about one on ones my all time favourite is hands down Lee vs Gaara, that fight is the reason I have continued to watch naruto. Any fight that has involved Lee have always been very entertaining, since they are always different and do not end because of the same technique every time.

Some other fights that have been good:
Orochimaru and company vs Sandaime (alot of cool techniques)
Shikamaru vs anyone (he is always fun to watch)
Most fights that haven't involved Naruto
All fights that have involved grown ups