Nice thread! I've not seen Supergirl yet but I'll give my thoughts on current seasons so far as others have!
Legends of Tomorrow S2
Actually quite dissapointing for me, on an episode by episode basis it was fine, but I felt the overarching story was a lot weaker this season, other than the episode that actually focussed on the legion almost entirely. Doomworld ended too quickly and the entire ending of the season seems nonesensical to me. The Spear of Destiny can rewrite reality, but it's also crap enough that people aren't completely wiped by it. Thus conveniently they can subconciously build a device that somehow gives you back memories that were WIPED FROM REALITY. Reality setting from a focus point, requiring time to fully set I can accept, the spear being crap enough to leave all the legends memories and personality from the original reality burried under the surface I can't. Let's not even talk about the villains leaving the legends alive, it doesn't matter if you have a character voice that it's a dumb idea, that doesn't make it any less stupid and nonesensical. It wasn't just with the spear they left the legends alive though... they had plenty of opportunities during the season to kill one of the legends, I think there was two seperate occasions where in one Damien had a legend injured on the floor and another where Rip had done the same, and instead of finishing them off they just left them injured. It's just not clever writing, "We don't want to kill off this character, but we want there to be some danger, so we'll just have the villain walk away... because... reasons."
Reverse Flash equally wasn't shown well, compared to pretty much all the Legends he's basically a god, yet there really was no sense of fear or power throughout the series in his character. Damien and Malcolm were relegated to lackeys for some episodes, seeming almost incompetent and being unable to aim a bloody gun. These are the same characters from Arrow that could go toe to toe with Oliver right? So yeah I guess my biggest issue with Season 2 was a weak overall story, with the writers not working around any of the issues in a clever way.
I still enjoyed it however, and while I am nervous about the storyline for next season giving the last scene of season 2... I'm looking forward to the next season.
Arrow S5 (so far)
Talk about a comeback! Prometheus is just an amazing villain, at first I was a little dissapointed at his actual identity, but now I'm happy that he's not someone we knew from a previous season. Phenomenal acting, the psychological warfare... episode 17. Sure there's some contradictary elements to Prometheus's philosophy and reasonings but they can be attributed to a not entirely stable mental state. Also thank god felicity and oliver are not together, though in that regards I am somewhat worried about what I've heard about episode 20, hopefully they put the final nail the coffin to that relationship and don't just open it up again.
I've not got as much to say about this yet as legends, the only big criticism is that the mid-season cliff-hanger was BS. Even the flashbacks are better then they've been since Season 2 at least! They need to nail the pay off at the end of the season, and this will be right up there with Season 1+2.
Flash S3 (so far)
Savitar... he's a better villain than Zoom. I want to say he's my favorite flash villain, but then you can't beat Tom Cavanagh Reverse Flash. Also my opinion on him may change completely if the last few episodes of the season and reveals about Savitar's identity and backstory really suck.
Not overly fond of Wally's story-arc this season, his costume makes him look like a wally too, HA GET IT!??!?!
I don't really know what to say until season 3 is over, but assuming they don't screw up really bad. Season 3 is better than Season 2 imo.
I'll also rank the seasons for each show just for the hell of it.
Flash: 1 > 3 > 2
Arrow: 2 > 1 > 5 > 3 > 4
Legends : 1 > 2