Because of the lack of good demographic information it is very difficult to say which country was most affected by the black death.  It came to Europe through Sicily and moved quickly to Italy.  High centers of population were hit particularly hard.  Florence lost over 65,000 people, probably more than 1/2 of its population. If you read the statistics you often see a staggering number of deaths in the Italian cities quoted, but to draw the conclusion that Italy was the hardest hit can be misleading, since Italy had the best statistical information it is frequently quoted.  English estimates say that their country lost 1.5 million inhabitants out of an estimated 4 million.  Whole villages and monasteries were lost while other villages were spared.  Estimates of deaths vary greatly from 1/4 if the population to over 1/2 of the population of Europe.  The number of deaths overflowed any recordkeeping of the times.  The dead were often just flung into pits and covered with lime, never being named or recorded.  Many times there were no funerals for the dead as the rest of their family's had died too, and priests and monks died in higher percentages than the rest since doing their duty put them in contact with those from the plague.  While most accounts of the plague focus on Europe, the plague came from Asia and we have little idea of how many deaths it caused there.  Below are some good sites to check.