A lot of people would probably disagree but for me it's France, because:
1.) It's ranked no 1 in terms of the number of visiting tourists per year. Making France the most common destination for tourist. For me it's like "Why go to a place where everybody else is going when there are so many "exotic" destinations elsewhere?
2.) I don't speak French and I think French language is difficult.
3.) General impression (from Quora actually) that French people hate/dislike or maybe just demand a lot from tourists. For example: tourist must be able to speak French in order to be "respected" or they must greet the store owners with some pleasantries in French before they serve people well see: One Café's Stand for Civility: Charging Rude Customers 5 Times More for Coffee
4.) General impression that French people make fun of people who speak French badly or even people who tries. (Opposite of how most other countries embrace/encourage foreigners who try to speak their language)
5.) General impression that it's exorbitant and expensive for something that's really "normal": Seeing Louvre, Eiffel tower, travel through Burgundy etc. I'd rather visit Bolivian salt flats, hike Machu picchu or see Petra.
I think one can go to France anytime. When one is old, frail and can't walk that much. There are many more rewarding destinations that one should visit while one is still strong and capable.