A lot of it depends on how you're going to use the space, and what resources you have available to you. Certainly as above suggests, a ground-source heat-pump is an excellent mechanism, but obviously you'll want to consider your usage. If you are going to have one person in a room or small area, you may want an isolated system with the ductless units to go with the larger overall system, and in some areas, well, a ground-source (or even water-source) system is highly impractical.

most efficient heating and cooling is geothermal... it uses a somewhat traditional electric heat pump but instead of utilizing ambient air, it uses a pump system to circulate water through pipework that is laid underground where the temperature is always about 50-60 degrees Fahrenheit. for a home like that, depending on the region, i would say that's your only real cost effective option. expensive to install... easy to maintain and your bill will significantly, less than gas oil and central air.

Like the others said, use a Geo unit. You can run zone controls to control the temps in each room.

If you had a very large open space, with smaller spaces like the home above (which im considering building) how would be the most energy efficent way to heat and cool both the small enclosed spaces and also as important, the big open large space.

I'd like to avoid swamp coolers since the walls are steel, and i hate humidity.

My initial thoughts are the most efficient roof mount unit for the size of space it needs to cool, open duct work (industrial look) with some of those compact heater/cooler units in each room.

Im no engineer, architect, or contractor so im hoping for some ideas.