I would say definitely Rock Lee vs Gaara is my all time favorite from the original series but if I'm going to be different, I would have to say the first fight between Zabuza and Kakashi.

Because that is the first time we see Kakashi's "Ability" We learn of his alias as the Copycat Ninja and in my head I always remember Sasuke looking at Kakashi when he finds out he has the Sharingan.

Oh my God, yes that slipped my mind. Chouji's battle was gut wrenching, how could I forget about it? I love Chouji and the Akimichi clan has always had a place in my heart.