Arrowverse viewing order:
This is the simplified list for easy viewing order for Arrowverse!
Webisodes (you can find them at youtube):
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 Supergirl Season 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
Comic books that also follow the series:
Best viewing order for the TV series Arrow, The Flash, Vixen, Constantine, DC's Legends of Tomorrow & Supergirl.
Webseries episodes can be found here:
Arrow: Blood Rush
The Flash: Chronicles of Cisco
Credit IMDb user koomy:
List of episodes I've watched in 2017.
New episodes: 882
Rewatched episodes: 457
RuPaul's Drag Race S5E02→S9
Rick and Morty S1→S3
F is for Family S1→S2
American Crime Story S1
Scream Queens S2E10
The Americans S1→S5E11
Star Trek: TNG S1→S5E09
The Walking Dead S7E09→S7E10
New Girl S6
Iron Fist S1E01→S1E02
Better Call Saul S3
Doctor Who S10E01→S10E07
Game of Thrones S6E06→S7
The Office S7E24→S9
Bill Nye Saves the World S1E01→S1E02
The Flash S2E20→S2E23
DC's Legends of Tomorrow S2→S3E06
Archer S1E01, S8
Parks and Recreation S1→S7
Twin Peaks S3
Unbreakable Kimmy Schmidt S3
House of Cards S5
Orange Is the New Black S5
Penny Dreadful S3
Mr. Robot S2
Peaky Blinders S3
Master of None S1→S2
Wet Hot American Summer S2
From Dusk till Dawn S3
The Defenders S1
2 Broke Girls S6E01→S6E16
That '70s Show S1→S4E05
Bojack Horseman S4
Agent Carter S1E01
The Deuce S1E01→S1E04
Star Trek: Discovery S1E01→S1E09
Once Upon a Time S5E12→S5E23
Band of Brothers S1E01→S1E04
Fear the Walking Dead S1
Big Mouth S1
How to Get Away with Murder S3E01→S3E07
Boardwalk Empire S1→S2
Arman ja Suomen rikosmysteerit S1E01→S1E07
Stranger Things S2
The Confession Tapes S1E01→S1E04
Kill Arman S1E01
Marvel's The Punisher S1E01→S1E05
Everything i have watched in 2017 so far in the order i watched it.
Here's an alphabetical list of what i have watched:
The 100 S4x01-S4x13
American Horror Story S1x01 - S01x06
Arrow S5x09 - S6x02
Chicago Fire S5x09-S5x18
Chicago Justice S1x01-S1x07
Chicago Med S2x09-S2x18
Chicago P.D S4x09 - S4x18
Designated Survivor S1x01-S1x14
The Flash (2014) S3x09 - S4x02
Frequency S1x01 - S1x13
Game of Thrones S1x01 - S7x07
Gotham S3x12 - S3x22
Homeland S3x01 - S5x02
House Of Cards S4x01 - S5x13
Legends Of Tomorrow S2x08 - S3x02
Lucifer S2x11 - S3x05
The Man in the High Castle S2x04 - S2x10
Marvel's Iron Fist S1x01-S1x02
Narcos S2x03 - S2x10
Preacher S2x01 - S2x07
Stranger Things S1x01-S1x08
Van Helsing S1x01-S1x10
The Walking Dead S1x01 - S3x11
This is a list with episodes of various DC Entertainment TV series in internal chronological order, according to this list: http://thetimelinesite.com/timelines/27-arrow-the-flash-tv-series-timeline.
2017-02-06: thetimelinesite.com seems to be undergoing maintenance at the moment (the above page has resulted in a 404 error for the past few days).
2017-02-13: thetimelinesite.com is working correctly again
2017-03-14: Updates from thetimelinesite.com incorporated into this list
2017-04-20: Updates from thetimelinesite.com incorporated into this list (Supergirl S02E15 till LoT S02E17)
2017-05-17: Updates from thetimelinesite.com incorporated into this list. Added Supergirl S02E18 till The Flash S03E20 and moved Arrow S05E17 and S05E18 and The Flash S03E18.
2017-05-29: Updates from thetimelinesite.com incorporated into this list. Added The Flash S02E21 till S03E23 and everything in between.
2017-10-27: Updates from thetimelinesite.com incorporated into this list: Added Legends of Tomorrow 3x01, 3x02 and all episodes in between (except for Arrow 6x02, of which the position is still uncertain).
This is a list with episodes of various DC Entertainment TV series in internal chronological order, according to this list: https://geekritique.net/2015/05/19/arrow-and-the-flash-chronological-order.
2017-10-27: The post on geekritique.net, which this list is based on, hasn't been updated in roughly half a year, so I consider it to be abandoned. Thus I advise anyone using this list to use my other Trakt Arrowverse list instead: https://trakt.tv/users/nidunc/lists/arrowverse-viewing-order-according-to-thetimelinesite-com.
Besides, the geekritique.net list didn't include all episodes in the first place (see the above bullet point for an example), while thetimelinesite.com does include all episodes (and is still being updated).
I will still be checking geekritique.net from time to time, but I presume it won't be updated in the future.