I would say US but life can get pretty boring and isolated here as well. I like the kinds of social intimacy and everyday contact with people in places such as latin america and south asia. you can drop in on friends and have some tea, leisurely discussions about politics and society, great street food (assuming your stomach can handle it), etc. major issue though is the trash typically strewn everywhere, unless you're living in one of those cleanly swept open-air malls such as singapore or dubai. but again, the messiness is in a way part of the appeal, also the incredible amounts of crowding.
i haven't traveled enough to actually pick a country, so i guess for now i'll just stick with the US both in terms of wide open spaces to roam and modern conveniences. ideally, however, i'd just keep traveling back and forth, for at least two+ weeks--none of this sleeping in hotels/airports nonsense--so that no place ever really gets boring or overwhelming.