Ryan Reynolds would have been my top choice for The Flash if it wasn't for him already playing Green Lantern. I always thought he was better suited for the role of Barry Allen, though I think with the right director/script he would have been very good as Green Lantern too.
Bradley Cooper (like him better as Hal Jordan), Chris Pine (like him better as Hal too) Ryan Gosling, Alex Pettyfer, Garret Hedlund, Scott Speedman, Charlie Hunnam, Cam Gigandet. I'd be fine with any of these, though Gigandet would be kind of a last resort. If I had my choice I'd probably go with Gosling as Flash and Cooper as Lantern. I'm not really a big Gosling fan but he's the best actor of the bunch and looks the part. Cooper fits Lantern more to me, though I think he could do either role well.
Personally I think they should try to fill out the rest main Justice League members (Flash, Lantern, Aquaman) with at least some what established actors, if not some bigger names. Not that they should turn away unknowns if they fit the role well but I want to see a well acted film. It's the first Justice League, something I've wanted to see for a long time, I want it to be good.
I really hope they don't connect the Arrow and Flash shows to the Justice League. I really don't like Grant Gustin as Flash. I also don't want anyone that young in any of these movies. Leave the High School version of Flash for the tween market of CW. I don't want to see that on the big screen.