To me, "'oughtn't you?" sounds awkward; "ought not you?" sounds extremely formal, old fashioned, and strange, and the remaining alternative, "ought you not?" although correct, sounds as if you are rebuking the person.
This awkwardness seems to be related to the idea that ought, unlike true modals, forces a to verb to follow it: compare with the equally awkward and strange, "You need to love your country, need not you?
Unless this is an examination question, I would avoid the tag question of "Subject ought to verb" and use "should ... shouldn't you."
"Oughtn't you" sounds fine to me, and I use it all the time. You ought to have been more careful, oughtn't you?
I think any awkwardness stems from the meaning of "You ought to love your country" - when we tell someone what to do it sounds a little odd to be asking for confirmation with a tag question. I suppose this is because I hear "You ought to love your country!" as a quasi imperative.
One ought to love one's country, oughtn't one? sounds formal but less odd to me.