My favorite arc was Sasuke retrieval. The thing that got me hooked on the series however was the complicated and weird relationship Naruto has with Sasuke. It turned out to not be so weird in the end, and their close-yet-distant brotherhood reminds me of the relationship I have with my brother. So I guess I like arcs where Naruto and Sasuke get to interact
Pein Invasion Arc
Sasuke Retrieval Arc
Kazekage Rescue Arc
There were moments/individual chapters from the War Arc and later that were very good, though