The must-watch episodes are 1-27, 29-56, 72-90, 113-143, 152-169, 172-175, 197-222, 243-256, 261-270, 272-278, 282-283, 296-302, 321-346, 362-375, 378-388, 391-393, 414-415, 417-421, 424-426, 451-463, 469-479, and 484-500. These storylines are taken directly from the manga.
Episodes that can be skipped without losing the storyline are 28, 170-171, 223-242, 257-260, 271, 279-281, 376-377, 416, 422-423, 427-450, and 480-483. Many of these episodes are standalones, mini-arcs, or flashbacks that have no relevance to the main story and often drag.
While filler, these episodes have good stories that should be watched including the legendary fighting monks in the Land of Fire monastery. They also introduce some new abilities such as a warrior being able to possess some tailed-beast chakra and turn into a partial jinchuriki.
These episodes about the leftovers from Orochimaru’s fallen empire can be skipped, but are worth watching for the amazing fighting and cool new jutsu. the story about how an “evil” Orochimaru disciple can change through love, and the Three-Tails that appear more than any other time in the series are the most noteworthy.
These episodes have the most interesting filler arc about a girl who has a forbidden jutsu sealed within her that can destroy villages, and the shinobi with a unique bubble jutsu who is tasked to keep her safe. Along with the unique jutsu, the arc has atomic bomb metaphors and the Six-Tails jinchuriki.