Even though Iron Giant is a death machine, and Iron Man is small and agile, I'll have to chose Optimus Prime. His transforming skills and overall epicness would make him win this tough fight.
I think by what we know about the characters - OptimusPrime would win. He has the most experience ACTUALLY killing stuff... the other 2 have never killed anything that we know of...
(I don't count any of iron mans wins as true kills)(and the giant is a big softy )
Iron man would win easily, or at least Tony Stark would... If any preparation time is given, Tony can make the necessary changes to his suit(s) in order to win the battle.
Iron man comic bada$$ version and movie version would both win. Prime is used to fighting things with a similarity to himself. and Giant is a big softie