Justin Timberlake
Okay hear me out. Justin Timberlake can definitely handle the lighter aspects of the character, but he’d be just as strong when it comes to the action scenes. He impresses more and more with each serious role, and with In Time he showed that he really can do some great action work. Reactions were mixed on that film, but Timberlake definitely killed it in that role.
The main thing that worries me about Timberlake is he may be too big of a name outside of acting for the general public to accept him in the role. While it really only matters how well he pulls off the character, and not at all what the masses think before the movie comes out, the big brass at Warner will be hesitant about doing anything controversial when it comes to kicking Justice League off.
Timberlake was on the shortlist for Green Lantern at one point, so you know he was on Warner’s radar and has at least some interest in taking on a comic-book film. He’d definitely make a better Flash than Lantern, so it’s good he wasn’t cast in that role. His casting is probably the riskiest of any actors I’ve named, but I think Timberlake would be excellent as Flash.