The fight between the 3rd Hokage and Orochimaru was insane. The fight happed in the following episodes:

Ep 71 : Absolutely Incomparable! A Hokage-Level Battle
Ep 72 : The Hokage`s Mistake - The Real Face Under The Mask
Ep 73 : Forbidden Master Technique! Shiki Fuujin

This was the first time he saw 3rd in action and he lived upto his name, taking on two previous Hokages. It was very sad to see him go. Fights like these made me love Naruto so much.