Jack, he's too important to the group for him to be killed off as the resident doctor.
Kate, to tough, and she's the hot factor for the show.
Sawyer, he will prove to be important as the show goes on, even though he will continue to butt heads with Jack.
Locke, his character is deep and he's the tough guy.
Charlie, there was talk he was the one that was going to die, but I think he's developed really well and his character is getting to be a much stronger person.
Sayid, he's got lots of background that could prove to be good filler for the show, considering he server in the Iraqi Republican Guard.
I think the one that dies will be one of these, Jin, Sun, Shannon, Michael, or Boone.
I could see them doing something with Michael and then having Jin and Sun sort of adopt Walt along with Locke, but that would be sad for the kid since he just met his father. Boone is expendable, and his involvement with Locke could lead to his death, especially once they open that hatch. Jin and Sun I think either could die and it might help with plot, if Jin dies then Sun would be free of his controlling attitude, if Sun dies Jin could develop more because of his loss.
Of course I'm probably wrong and it will be Hurley or something, although there hasn't been a "Hurley" episode yet, I guess that one's coming up.
On a side note, with Lost, Alias, and Felicity JJ has entertained me for like 6 years. His shows have continued to be some of my favorites to watch. When is he going to get a REAL SHOT on the Big Screen?