In my opinion, its more fun to watch all the shows. I have not watched <PERSON> and <PERSON>, but I'm considering doing so. I am not sure Flash (1990) is related.
As was stated by another person, watching all shows helps you better understand what's happening with all the characters; however, as one of my professors told me, you don't HAVE to do anything. I advocate for at least watching the 3 mains. I watch the 4, as stated above. I like Arrow and the <PERSON>. Also, I ship <PERSON> and <PERSON> as besties. :) They need to make more Arrow/The Flash crossovers in my opinion.
Legends, is ok. I can understand not wanting to watch it. But is more fun compared to Arrow's darkness and <PERSON> kind of starting to channel <PERSON> in The Flash season 2 and, admittedly more so the new <PERSON>, again now in season 3. 282
Its Doctor Who in the DC verse. Fun facts: the <PERSON> comics predate Doctor Who, and <PERSON> is played by the same actor who played <PERSON>.
Supergirl is, at times, very much a liberal setpiece. That is not always bad. As a female who likes superheroes, I very much like how they are trying to get rid of the fact that most female superheroes were written by men who were exercising some fantasy when making their costumes. Also, the fact that most female superheroes seemed to be made as a counter to a male superhero. It goes a little too far in the left-ward direction sometimes, but mostly, it goes just enough to make me happy. Also, it is one of the few recent shows I have seen with a female lead where they got rid of the love-triangle angle. That alone makes it awesome to me. The crossover episode with the <PERSON> in Supergirl 1x18 was super adorable. And sure, it was kind of played off that <PERSON> was used to make <PERSON> jealous, but it could also be seen as someone who had been doing this a little longer offering advice, and also gave <PERSON> and <PERSON> a chance to play around with their powers.
If you decide to watch the 4, looking at the release orders is the best indicator. The timeline doesn't always seem to match up, but enough so you can keep up. 282