"Superman", I mean why not? Jeez...
Of course, if there was another title I would choose from, it would be naming the series finale after Smallville's main theme: "Destiny".
But I hope it is "Superman", because I still feel, and they should, and they have to, have Superman in the final episode in costume, period. I mean, they better.
I should say though, if the series finale happens to be a 2-parter, kinda like a movie like the Smallville/JSA was, then part 1 should be "Destiny" and part 2 should be "Superman". The marketing for the entire thing will ultimately be "Smallville: The Series Finale" or "Smallville: Destiny (Destiny's) Fullfilled" or something like that.