This is the simplified list for easy viewing order for Arrowverse!
DC's Legends of Tomorrow
Freedom Fighters: The Ray
Constantine: City of Demons
Webisodes(you can find most of these videos at YouTube or at CWtv website):
Blood Rush - between Arrow S2E05 and S2E06.
Chronicles of Cisco - between Flash S2E17 and S2E18.
Superhero Fight Club 1.0 - is a promo for the final episodes of Arrow S3 and Flash S1 (arrives after Flash S1E18)
Superhero Fight Club 2.0 - is more or less a promo for Supergirl joining CW. (arrives before Flash season 3)
Constantine - watch before Season 1 of Constantine:
Modern Mystery: Tales of the Dark Arts - Who is John Constantine?
Ravenscar Institute Patient Debrief
The Flash - Stretched Scene #1 : Flash S4E06
The Flash - Stretched Scene #2 : Flash S4E07
The Flash - Stretched Scene #3 : Flash S4E09(there's also a deleted scene for this episode)
Constantine has a cameo in Arrow S4E05. I do recommend you to watch this series, but there's no must. But he might get back into the Arrowverse one day. (and I also would recommend to watch it in this order: 1, 6, 2-5, 7-13. Episode 6 was shot to air as the second episode of the series)
I have added SupergirlSeason 1 before the crossover episode to the list. You can watch it basically whenever you want, as long as you watch it before Flash S2E18 or after Flash S2E18(I personally prefer to watch it before Flash episode, but both ways work). Also S1E04 and S1E05 were aired ouut of original order. Episode 5 should be viewed before episode 4
Constantine: City of Demons: So it appears that Constantine: City of Demons is not a continuation of the Constantine series and so it is not included in Arrowverse. Instead it's in the same universe as Justice League Dark (so it's in DC Animated Universe). But for now I'm going to leave it in the list, as it still explains things like "Newcastle" and etc. But they may get connected, as Constantine will becomes a regular on DC: Legends next season. So we will wait and see.
Comic books that also follow the series:
Arrow - takes place turning Arrow season 1
Arrow - Vengeance - takes place turning Arrow season 2
Arrow: Season 2.5 takes place between Arrow season 2 and 3
Arrow: The Dark Archer - takes place at the end of season 3 of Arrow
Arrow: A Generation of Vipers - takes place before Arrow S4E07
Arrow: Fatal Legacies - the untold story between Arrow Season 5-6.
The Flash: Season Zero - takes place turning Flash season 1
The Flash: The Haunting of Barry Allen - takes place after Flash S2E07
The Flash: Hocus Pocus - Non-Flashpoint Timeline (Flash season 3)
The Flash: Johnny Quick - Flash season 3 (Non-Flashpoint Timeline)
S.T.A.R. Labs: Cisco Ramon's Journal – May 1, 2018 (pre-Flashpoint)
The Adventures of Supergirl - takes place turning Supergirl season 1
Sister Act - takes place before Supergirl S1E04.
Supergirl: Age of Atlantis - towards at end of season 2
Supergirl: Curse of the Ancients - towards at end of season 2