A former talent scout's opinion: Syesha's issue is, while she has a great "Whitney Houston" voice, she struggles with other genres. Jason Castro, well, has a nice voice (as do all the final 12, in their own way), but can't hang with the three other contestants left.
David Archuleta has a very well developed and versatile voice that is only going to improve with age, so from my standpoint, has the most potential for stardom.
David Cook has a great voice and has more ability to cross genres than the other contestants left, but seems to be much more comfortable in the rocker-dude role. It will be interesting to see how he does with showtunes.
Right now, I'd most like to represent Archuleta, with Cook right behind him.... big bucks, big bucks.