I would love number 7 or 2 especcially if in two you could manually fly your ship between moons and there were aliens that had faction wars between the moons so moon 3 vs moon 42 so its like hive vs fallen if you know what I mean
EDIT: Apparently Neptune, Saturn and Uranus are gas planets so I don't need to know a 318th time thanks.
Europa concept art: http://destiny.mmmos.com/?page=view&id=1750&title=europa-concept-art-is-beautiful
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Saturn's Giant moon Titan. Home of the Titans. Best planet for the best class.