For Superman, I needed someone who is tall and strong but doesn't look like he's in high school. Most of you have probably seen the " World's Finest" fan trailer by Sandy Collora. In it, their best casting was of Superman. I cast Mike O'Hearn for Superman again.
Batman needs to look like he's had a dark past. I've seen a lot of users cast Adrian Pasdar for Batman. I think it would work out great if he can fix up his voice.
Wonder Woman needs to be fairly unknown. Jodi Lyn O'Keefe is too overused on this site. Most of you have probably also seen the fan film "Grayson". In it, Kate Clark does an excellent job as Wonder Woman. Why not get her again?
A few years ago, the rapper Common was rumored to be cast as Green Lantern. I would have cast Samuel Jackson but he's already Nick Fury. Common looks the part and has talked about it on IGN.com
Most people want Ryan Kwantan, Chris Evans or Ryan Reynolds for Flash. Evans is Captain America, Reynolds is Green Lantern and Deadpool and I'm not sure about Kwantan's voice. Michael Rosenbaum voiced him in the animated series and with some hair dye, he could be perfect.
Maria Canals-Barrera was the voice of Hawkgirl on the TV series and even looks like the character.
Martian Manhunter is a tricky one. Dwayne Johnson looks the part but I've gotten used to Carl Lumbly's voice. So I decided, why not both? Now tell me this wouldn't be awesome.
Sorry about the picture, but it was the only one I could find with Johnson as Martian Manhunter.