Ok this is my big theory for the end of the final season..and I guess thats means im going to be very wrong. In the end however I do think the numbers are the key to everything. According to what we have learned the numbers are a sequence which predicts the end of the world. We know now that the numbers have been assigned to each one off the remaining candidates.
If we take all of what we know so far about the numbers my theory is that the people who represent those key numbers will cause or help cause the end of the world. Which to me means smokey will win in the end.(Locke allowing himself to be used by smokey will be his contribution. Now this does not mean I think its gonna be a sad ending. If you look at the FST it seems the numbers are no longer negative. So it may just mean that in one universe there will be a happy ending and in one universe an unhappy ending.
Desmond being the variable that throws of the equation has the oppurtunity to change these two outcomes and he is trying to do now. In the end however I think he will fail