The first two seasons of Arrow and the first season of Flash are genuinely pretty good. Season two onwards of Legends of Tomorrow is amazing and ridiculous
They're guilty, cheesy pleasures that are CW shows more than anything else.
It's both of these. I thought that Arrow season one was legit awesome and marathoned through it pretty quickly, which is something I don't usually do. Flash season one is also pretty good and the way it leads into season two is a comic nerd's dream. The seasons that follow have their ups and downs. My biggest problem with Arrow and Flash in particular are that too many seasons' main villains are like evil versions of them. How many evil archers did we really need? Why is every Flash villain also a speedster? HOLY FUCK STOP.

Legends Of Tomorrow takes a season to find its footing and the cast changes up a bit, but it gets pretty fun with them going to different time periods and related fuckery with villains who might be dead on Arrow or Flash popping up because it's the past.
Right now I'm still on the last seasons of these shows- have two more episodes of Legends, finished Flash (kind of a mixed season), and am halfway done on Arrow, which is pretty awful, but I want to watch it all anyway and get to the current season, which is apparently an improvement.
Supergirl is now part of the Arrowverse (season one was not), but takes place on an alternate earth. I haven't really watched it apart from the crossover episodes. They've done one a year for the past three years now. They're pretty dumb, but it's fun to watch everyone together.
Another note about Arrow- he's basically Batman. As in they even bring in Batman characters like Ra's Al Ghul and have a lot of storyline with his family, Lazarus pits, the league of assassins, and all that stuff. You'll also get a ridiculous amount of back story that gets to the point where it feels like a hundred people were on the island that Ollie was shipwrecked on before he became the Green Arrow.

And finally, some cast members just don't go away and might pop up on all three core shows- some people play multiple versions of the same characters and in one season you might see like three versions of them because of flashbacks, alternate earths, or both. It can get a little nutty but it's all part of the cheese. Tom Cavanagh, who's on the Flash, plays a different version of his character Harrison Wells every season and it's actually pretty awesome. You can tell he's having fun doing it.