Italy or Spain since there fruit and vegetables are very inexpensive .Plus they have limited fast food restaurants and soda machines are difficult to find
Fiji (though, do not get injured, because it is not easy to find a doctor nearby).
I wonder how many countries have you ever stayed in for a long period of time? Perhaps, you should go to Afghanistan or Iraq for a while; you would be begging to come back here after couple of days.
There is no such thing as a healthy country. It's healthy choices. It shouldn't matter what country you're living unless it's a very poor third world country. Other than that you have access to healthy foods. Yes, the USA has unhealthy foods, but healthy food is still accessible. Just as every other country.