I feel they should make a Wonder Woman movie before Justice League. Batman and Superman are pretty much standard fair at this point (plus Man of Steel is coming out next year anyway), and Green Lantern just had a movie, so he's good. Flash can be handled with some dialog or a quick flashback (no pun intended) in the movie explaining his origins.
A Justice League movie could be good, but needs to be handled properly. Just a "Hey, Avengers worked, so Justice League should work too!" isn't necessarily the case, as the reason Avengers worked was because the movie was well made and it was built up to appropriately (5 movies and all that, 4 of which were decent at worst); basically, it needs to be handled in a way that doesn't scream "Its DC's attempt to counter Marvel's Avengers success!" the way Green Lantern felt like it was DC trying to promote a non-Batman/Superman hero, in a response to Marvel getting all their Avenger heroes spotlights (face it, before the movies, was Iron Man really that mainstream outside of comic readers? I'm not saying "obscure, not well known", more like how many people knew that "Tony Stark' was the man behind the armor, for example?