After seeing him in The Hangover, I think Bradley Cooper is definitely my pick of the three mentioned, which is admittedly a woeful list to begin with. Ryan Reynolds, while exuding the natural charisma required for Hal Jordan, is a bit too witty of a choice, and plus, he's already Deadpool! That and I think he'd make a great Flash, too.
Justin Timberlake? Why the hell is he even being considered, nevertheless in the running?! That would be just as bad as if Ashton Kutcher became Batman like all of those premature casting rumors circulating Batman Begins back in the summer of 2003.
But Cooper... He just has natural screen presence. He can be fun and playful when required, but I have this strange feeling he probably has the most dramatic gravitas out of those three. I can't quite explain it.
Cooper isn't my ideal choice for Hal, mind you. That would be Nathan Fillion. But out of those three mentioned, Cooper is the clear winner. I'm surprised there's even a contest.