In which country is Buddhism the least influential today?

Did You maybe mean "the most" or did you forget the options?

I would say that among all the countries in the world, It would be least influential in countries with state religion (Vatican, Malta, Iran, others) and among those I would say that we can be quite sure that there are no Buddhists in the Vatican City (but they pope and Dalai Lama often meet) - so this would be a good option.