Ultron, if my Marvel knowledge is correct, starts out as being able to destroy a planet or wipe out the surface of one with nuclear weapons. Later he gets much stronger and kills all the Avengers in a stronger state.
Unicron, as it is well known, is usually a planet devourer. However, I hear he is also capable of devouring Universes/dimensions in some forms.
Ultron vs Unicron in planet destroying form, then Ultron vs Unicron in their strongest forms.