For me it was after Naruto gained his kyubi mode in the war arc. The war arc started of really good and interesting with some mysteries,tactics and naruto gaining kyubi mode was great. However after naruto gained the kyubi mode the war started to drag on with too much useles edo tensei fights. The excitement started to lack at this point.
The only time it went down for me in the first half was in the Search for Tsunade arc since it was so slow, Sasuke wasn't in it and the plot felt a bit aimless. However, it was meant to develop Naruto's new technique and Jiraiya and Tsunade's characters so it grew on me, though still isn't my favorite to this day.
In part 2, It went down a little for me in the Sai and Sasuke arc since there was virtually no plot progression lols and focused mostly on developing sai, but shot up quickly again the following arcs. It went down again for me in the war arc with the seemingly rushed edo tensei fights but went back up when the fights with edo Madara, Kabuto and Obito began. The juubi arc was a bit slow and I was disappointed with Madara being defeated like that after so much hype and development with him, not to mention he finally getting both the rinnegans back, as well as Kaguya's character not being developed much, but the fights, characterizations and drama were still great.
