Before I watched the finale, I most definitely ruled out Sydney. There was no way that the creators could kill Sydney off. I expected that Sloane would be killed, because he's been the antagonist through most of the episodes. Through some way, Jack would sacrifice his life to protect Sydney or her child, although, I did not expect his sacrifice to be in the form it came in, which, I must admit was very pleasing.
Nadia, Irina and Vaughn did not come to mind, when thinking about the deaths. Nadia had already died. The creators would need to have at least one happy ending, and without Vaughn and Sydney, that would not be able to happen. Irina, I thought she would succeed with whatever she was trying to do.
However, the -main- death would have to be Sloane.