For both the original Naruto series and Shippuuden, which episodes should I watch if I want to avoid the filler? I'm only interested in core plot and character developments.
To make this question a bit more concrete: Which episodes are directly based off the manga, and which were produced with unique content for only the anime? At what points did the anime "catch up" to the manga and require non-manga-related content to continue airing episodes while the manga produced new content?
The Naruto anime is broken up into two "Series". The first is Naruto, which covers events up until the 3 year time jump in the manga. The second is Naruto: Shippūden, which starts after that jump and continues onwards. Both series have a fair share of anime exclusive plots, so here's the breakdown of Anime exclusive plots:
Naruto
Naruto and Jiraiya leave the village for training, and others begin training as well
Naruto: Shippūden
Also of note is the Kakashi backstory "Kakashi Gaiden" is shown in the Shippūden (Ep 119 - 120), which is out of order as to how it appeared in the Manga (Chp 239 - 244). In the manga, it was between what became the first and second series (the time jump).
Additionally, none of the movies are based on Manga plots.
Source for most of this information was the Naruto Wiki. "Shadow of ANBU" info from Naruto HQ
Naruto Shippuden's story has progressed a lot since from the last answer. The current filler episodes are:
57-71, 90-112, 127-128, 144-151, 170-171, 176-196, 223-242, 257-260, 271, 279-281, 284-295, 303-320, 347-361, 376-377, 388-390, 394-413, 416-417, 419, 422-423, 427-442
TL;DR there's no one filler list that 100% agrees with each other, viewer discretion is advised.
Combining known sites for their list of filler episodes:
here are the greediest/most filler episodes: ("plain" means "totally filler" | "italics" means "mostly filler", prioritized)
