I think most of you are way too harsh. Just because something's not your cup of tea, it doesn't mean it's trash.
Anyway. I've watched them all and I have to say for me it's:
Gotham: easily the darkest and best of all DC shows. It even rivals DD and JJ (Luke Cage is just boring imo). Great characters, great stories, great acting. The procedural formula somehow works.
Arrow: Very good show, though at times it can get ridiculous. The most annoying part is someone getting a bit of training and beating two trained assassins at the same time. Other than that (and some romantic bs that are just badly written) it's great.
Flash: a very fun show which was super easy to watch. They made some changes and made it darker which doesn't really work for it(the actors can't pull it off), but we'll see. The first season was fun, but the guy who plays Dr. Wells does a terrible job.
Supergirl: surprisingly good. Not great, not incredible or anything but it sure is charming. The lead actress is perfect for the role and I can't help but root for her. The rest of the cast is good too. Just like The Flash, it's really fun to watch.
Legends of Tomorrow: had tremendous potential and could have been my favourite, but the acting of several people has been atrocious. Even good actors like Wentworth Miller do a pretty bad job (I take it he's instructed to act like this. Too over the top for me). The fights and special effects are great. There are a ton of good elements there. They just need to fix a great number of things, like the terrible villain they had the first season and the story being a bit more tight. Because as it is it's hard to get attached to anyone. I heard the second season is already much better.
