Flash >>>>>>>>>>>>>> S2 Arrow > Constantine > rest of Arrow >>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Gotham
Flash is the only of the four series with absolute faith in its premise, and it shows. It's not ashamed of being a comic book television show. It's not afraid of super powers, garish costumes, cheesy names, talking gorillas, time travel, or anything else that makes comic books fun to read. They decided to make a Flash show all the way from the outset. The 90s Flash show is really a Burton Batman show for the first ten episodes, and by the time they figured out what the audience wanted it was too late to save it. I remember it fondly, though.
Constantine has too many procedural elements, and it's a fairly watered down interpretation of Hellblazer. And it stinks of David Goyer.
Arrow is a wannabe Batman show with some Smallville teen angst thrown in (though it's still orders of magnitude better than Smallville). Season one of Arrow was really bad until the midseason premiere. Season two and the second half of season one were great, but season three has been a mess so far.
Gotham can't decide whether it wants to be a police procedural starring Commissioner Gordon or a half-assed Batman+enemies prequel, so it aims for a middle ground and fails spectacularly. The set design is really good and Donal Logue is a good actor, but I can't think of anything else about the show that is even close to mediocre, let alone good.
