This is DCTV related (not DC Extended Universe), but on Netflix, I've been working through Season 2 of The Flash, Season 4 of Arrow, and Season 1 of Supergirl. I'm following the timeline of things in watching all of them and am about on episode 10 on each one. Based on the timeline, I'll soon be jumping into Legends of Tomorrow as well. With that said, here are my thoughts so far on each one:
Arrow (Season 4): Season 3 was when things started to go downhill by adding too many superheroes and too many bad ones at that. Not everyone needs to become a superhero. Laurel Lance (Black Canary), Red Arrow, and Speedy are all just straight-up horrible in their parts and drag the show down. The only ones other than the Green Arrow I like are Diggle and Sara Lance (White Canary). Season 4 so far is just more of the same, only with going too overboard on the magic thing, which really loses the groundedness of the show that made it once great (Season 2 was the peak). All said and done, this season is really the worst one so far, but it is good to hear that they do start to turn things back around in Season 5.
Flash (Season 2): Great show with a continuation of what made the first season so great, which is good characters, with great actors, with a ton of awesome drama and heart. This is what Arrow once had (not quite to this level though), and has lost. This is definitely the best show of them all still and maybe the best comic book TV show out there right now.
Supergirl (Season 1): This was a show that seemed to get mixed reviews, so I came in a bit skeptical. What I found was a new show that was a lot better than I expected. I'm loving pretty much all the characters so far, particularly Melissa Benoist as Supergirl, Jimmy Olsen, and really loving Maxwell Lord as the Lex Luthor type villain. Yes, the show is a bit heavy-handed with its girl-power simple girl in the big city, under the big lights type thing. However, if you can get past that, there is some good messages with some good drama intertwined in there. Looking forward to Season 2 already with things switched over the the CW, which I think can only be good for this show moving forward.
In the end, if I had to rank them, I'd do Flash at #1, Supergirl at #2 and Arrow at a distant #3.