Depends on the story and the used artwork ....
1. Starting with a young Kara who's discovered by Superman (like the S:TAS) I'd go with Ariell Kebbel
2. Staring with a rather mature Kara with an origin like the old movie I'd go with Sarah Carter
(Note: the pics are hosted on hidebehind.com - you may need to turn off your firewall to see them)