F800GS all the way.
The F700 is plenty capable on road (more so than the other two) because of the wheels and the height. You can farkle it up just like the F800GS so it'll take you from A to B on pavement w/o a problem.
But when you get off road, those cast wheels cause a little bit of concern for me. If you run into a nice pot hole and dent the rim, you're out because of the lack of air. Solution? Pack a tube just in case.
The 1200GS is simply a big and heavy bike. I couldn't imagine dropping it on a hill and trying to right that thing more than once. It's a great bike on the road but off road I'd be really concerned about the additional weight if I didn't need it.
If you had a passenger, then they'd be there to help lift, but otherwise it's an endeavor and would turn a ride into a fear and that fear could cascade into a less than enjoyable experience.
One person, with gear for a few days? The F700 and F800 GS have plenty of engine to do it.
I would get the F700GS if I planned on abusing it and would be shopping for a new bike in 2-3 years. I'd get the 1200GS if I was planning for the 5 year time frame. The F800GS otherwise.