1. Chunin Exam Arc (All tests were pretty fun. Orochimaru's debut with the curse mark, and a showcase of many secondary characters. A series of fights is what a lot of people were urging for. Lee vs. Gaara, Naruto vs. Neji... just so many standoffs. Not to mention Jiraiya's debut)
2. Sasuke Retrieval Arc (Sound 4 were awesome enemies, followed by Kimimaro and finally the VoTE battle. The Sand Siblings stepping in was amazing!)
3. Itachi Pursuit Arc (Jiraiya vs. Pain AND Sasuke vs. Itachi? Nuff said. Jiraiya's death and Itachi's revelations = some of the most heartbreaking moments in Naruto for me. On a side note we got Deidara vs. Sasuke, plus both Tobi and Kabuto were revealed to have a stronger stand in the plot. Although the squad never did much in the end which disappointed me)
4. Invasion of Pain Arc (Konoha is attacked again at home field. The big cheese of the Akatsuki goes on a rampage and even eradicates the village. Sage Mode Naruto makes an appearance and almost breaks the seal. Then Minato, the legendary Fourth, shows up!)
5. Land of Waves Arc (The first real threat shows up and wow they're a good start. I'm sure most people know why).