Theory 1:
Simply, Tony Stark creates Ultron. I feel Iron Man 3 was all that is needed for evidence on why he could be the creator.
Theory 2:
A more thought out explanation, SHIELD creates Ultron. Kevin Feige has stated that Captain America: The Winter Soldier will have the most build up for Avengers 2. From what we can speculate, Cap2 is about rogue SHIELD agents. Most important is evidence presented in Avengers 1. Nick Fury, when Cap finds the Tesseract weapons, explains the Avengers are "Phase One" and said weapons are "Phase Two." This leads me to believe that Ultron will be created by SHIELD to prevent what hapened in New York. Finally, Iron Man 3 showed that the armors were on their way to being free thinking. I believe Ultron will be a Tesseract powered Iron Man armor with free thinking AI who is either corrupted by rogue SHIELD agents, or they keep that he had to protect humans from humans. Tony might very well be the creator, but if its just SHIELD tech gone wrong, that at least leaves the possibility that Pym is involved somehow.
What do y'all think?