Watch: Episodes 1-26
Watch: Episodes 1-24. This is a sequel to the main Higurashi series
Optional: This is a side story that gives a bit of background to Hinamizawa village. I found it ok.
Skip: These are short DVD specials that don't add to the plot.
Optional: This season gives us a scenario where Rika is sent to an alternate Hinamizawa, where they did not win the Dam War.
Optional: Wasn't a fan of this season.
Episode 1: Wayyyy too much fanservice. Seriously, only watch this if you want the FBI to pay you a visit.
Episodes 2-3: These episodes involve magical girls. These episodes were okay, but didn't add to the plot.
Episode 4: A younger Rika travels to the future. This episode was also okay. Enjoyed it more than the first three episodes
Optional: Alternate pandemic scenario in Hinamizawa. Personally, it wasn't my favorite.
It feels like a remake at first. The timelines are very similar in the first half of the anime, but it does eventually continue the story. Watch the original first, as it references the ending to Kai.
If you don't want to repeat some timelines you can skip episodes 1-13, though it does contain buildup to the second half, despite the repetitiveness.
Watch episodes 14-24. That's when the story starts to continure.
I didn't enjoy this as much as the main series
Watch: This is a sequel to Gou.
Episodes 1-11 replay the previous seasons timeline from Satoko's perspective.
Episodes 12-15 have the newer content and ending.
The ending felt kinda rushed though, so be warned