Since the reveal of the upcoming Man Of Steel Sequel, which many have simply referring to as Batman & Superman or The Worlds Finest, there has clearly been a big amount of speculation on what the film will be called.
But it seems the films writer David S Goyer may have already made the reveal for the films name (thanks to ComicBook.com for the quote) :
“So over in Hall H at the Warner Bros.–I guess most people know now–at the end of the Warner Bros. panel, Zack came out and he brought Harry Lennix, who plays General Swanwick in Man of Steel and we–actually, Harry read a bit from [The Dark Knight Returns] and so the next film we’re making, we’re already in pre-production, comes out in summer of 2015 and it’s–we’re actually not sure whether the title is Superman vs. Batman or Batman vs. Superman but yes, it’s–that rematch, that combination, the two guys onscreen and that’s happening.”
So there you have it. While many (including myself) prefer the idea of “The Worlds Finest” or even simply Batman and Superman, it seems that the writer himself wanted to title the film with the idea of the two characters going head to head.
In the end, its not up to the writer but DC, Warner Bros, and even the director Zack Snyder. But only time will tell, but odds are we wont see the films official title until we get a trailer sometime late next year.