The first season of Daredevil was great and certainly worth a watch, even if you don’t bother with the rest. It is considerably more violent than Agents of SHIELD, however, so be warned. The violence somewhat diminishes and becomes more comic-booky as the shows progress.
Jessica Jones starts off very strong but the story is dragged out for too long, so overall it is just okay. The character comes with a lot of heavy psychological and emotional baggage and the show does not shy away from that.
Daredevil season two seems fairly divisive. I thought it was a massive letdown from the first season and saw Netflix Marvel settle for mediocrity rather than aiming for greatness, as the generic comic book tropes and some pretty woeful writing crop up. Others enjoyed it.
Luke Cage is pretty much universally regarded as being quite good for about the first half of the series and then inane drivel for the second half.
I can’t say anything more about Iron Fist. It’s like kicking a puppy at this point.
I’m not sure Defenders would make much sense unless you’ve seen at least the Daredevil and Iron Fist shows.
So, I would recommend going for Daredevil season one to start and then check out the first episodes of the other shows. It doesn’t really matter that much which order you might watch the rest of them in.