Best Arc's
1. The Land of Wave
The tension between the main trio, wrangled by their teacher Kakashi, is already rampant and there is an exciting duel with the mysterious Haku and the fierce Zabuza. The tailed beast forms and Sharingan are glimpsed here, but not fully explained which hints at what's to come. All in all, the arc is low stakes compared to others, but a great start for the series.
2. Sasuke Recovery
Also known as the Sasuke Retrieval arc, this arc marked a turning point for Naruto, both the character and series. The story had its dark moments, but that darkness would amplify here. Sasuke had left the village of Konoha, and Naruto forms a group to get him back. This arc marks the debut of Naruto's promise that would carry until the end of the series.
3. The Five Kage Summit
In direct contrast to the Land of Waves' simplicity, The Five Kage Summit arc is dripping in complexity. The leaders of many great ninja nations come together to discuss what to do with the Akatsuki. This results in an arc full of political turmoil. It does a great job of expanding the world; bringing in characters previously only mentioned.
4. The Akatsuki Supression
The Akatsuki Suppression Mission is a more understated arc in Naruto Shippuden. It comes between an underwhelming arc and a plot-critical arc and concerns itself mainly with a battle with Hidan and Kakuzu, who aren't among the organization's most notable members. However, the arc comes with a lot of emotional weight as well.
5. The Fourth Shinobi World war
The Fourth Shinobi World War is actually several arcs in one. Some would even consider the ending arcs, like the reveal of Kaguya, to be part of this arc. This is because the Shinobi World War is the defining climax of the story that everything had been leading up to. The biggest appeal of the arc for fans is at the beginning and end.