“the place with the most Kosher restaurants is not NYC , Miami or Jerusalem, Its Paris “
This is correct. And I was unable to find a kosher restaurant in Paris with bad food!
“The problem with food businesses is Shabboses and Peseach. I think most kosher Donuts are not owned by jews. so these are not problems for them.”
I am unaware of any Dunkin Donuts, kosher or otherwise, that is not open every day of the year. There are halachic workarounds for this involving a non-Jewish partner — consult a good rav with yadin yadin to work that out.