I mean making Oxygen is not so hard Afterall Water is called H2O and most of everything passing by Jupiter is made of rocky ice surfaces now I think food is gonna be the hardest thing to make now your question dealing with the moons of The gas giants which have like heated cores that make Inner oceans possible i think those ones would be THE HOLY GRAIL of the solar system, Now if you deal with mars i think that place is more or less the new Industrial heartland of the solar system because of (INSERT THE NUMBER OF PROJECTS TO SEND MAN TO MARS HERE)