I enjoy Flash, Arrow and Gotham. Constantine lost me at the second episode.
Of them all I think the Flash is the best, although Arrow is a close second.
All three shows have good casts, but Amell and Gustin are much more likeable than the guy who plays Jim Gordon (the rest of the Gotham cast are great) but Gordon himself is a bit boring.
The real stand-out is of course the Penguin, he's taken Batman's lamest enemy and made him genuinely creepy, menacing and interesting. The Dons are a bit dull, but Bullock and Alfred keep things moving along, and Jada Pinkett does crazy so well ! I would say that Gotham's had some absolutely great episodes and some that feel like filler.
I find Arrow has great moments, but tends to balance those with real cheesy moments. Just when something kick ass has happened Arrow makes some lame speech - and the voice is silly. Otherwise, the show has great action, and the cast have a nice chemistry.
Flash is the most consistent of the three, the developing sub- plot with Wells/Reverse Flash is great, and the other comic book characters have received good treatment.
Gustin has completely surprised me as the lead, he's likeable and you can't help cheering for him. I kind of wish they'd get a bit more consistent with the Flash's powers. Sure he can bit hit by surprise, but something he can see coming ?
Anyway, while the Arrow team have good chemistry the Flash team (Cisco, Caitlin, Joe and Wells) actually feel like a little family. Iris is a bit annoying but otherwise they're a fun bunch, and it's great to see John Wesley Shipp in there.
Also, the depiction of super-powers has been reasonably good.
So in terms of ratings. Flash 9/10 Arrow 8/10
Gotham 7.5/10 (average, some of the episodes, like the one where Bruce and Selina go on the run from the assassins are just magic, but there are a few that feel like they're just padding out the season).
Having said all that, any of those shows are more fun to watch than Agents of Shield or Agent Carter, even at their best.
I still don't know why Marvel makes tv shows about relative no-body's while
DC/WB/CW makes shows about more famous and well established characters.