I'm on the Christina Hendricks bandwagon. For a slightly left-field choice, how about Lucy Lawless? She's played a similar role before, obviously, she can act (see her turn on Battlestar Galactica) and she's got the right look for it.
I am sticking with and reiterating my support for Bridget Regan as Wonder Woman.
First she clearly looks the part.
Second she can in fact act.
Third as demonstrated in Legend of the Seeker, she has moves, she could probably out kick Diana Riggs in her prime.