Hard to say. Naruto mostly kind of feels more like one huge, connected story rather than several arcs. Well, I pretty much liked the arc after Konoha's invasion (from Itachi's first appearence to Tsunade becoming Hokage), the Sasuke retrival arc, the Gaara retrival arc and the Itachi arc (everything from Sasuke's fight with Orochimaru to Sasuke's tears and his Mangekyou Sharingan). The war arc is pretty awesome too, just like the Gokage Kaidan arc was. Well, I just love every arc there is.