I still believe Siobhan is the most marketable of all remaining. She has a great look and voice and with professional training (which she will get) could be a huge star. She has natural born talent. She is a flake (or is in Siobhan land, as you put it)...but so are 99.9% of talented artists. I believe she has Carrie Underwood potential with the right handling. Whether she wins or not really doesn't matter...she will be given a recording contract without a doubt.
Crystal is very good also, but probably fits into the Rock or Blues genre... doesn't have the "look" for the lucrative POP market, IMHO.
Lee is pretty good and reminds me a little of David Cook without the personality. He doesn't, however, have the vocal range or stage presence of David Cook.
Looks like the new Tonight Show band leader is going to be Rickey Minor, who will be leaving Idol at the end of the season.