Undoubtedly the prestigious FIFA world cup.
The uniqueness and scarcity of the tournament makes it really tough.

There are lots of reasons it is difficult to win, like;

• Hard Qualifying Stages
  The first thing I would like to start with is how hard it is to qualify for the world cup. A team will have to participate and battle other teams to be able to get a spot in the world cup tournament.
  For example in Africa, teams are divided 4 each in different group, and only one team can qualify, After that one teams qualify, another knockout matches will be played by the teams to know the few who will be playing at the world cup.

• Level of Competition
  The level of Competition in the tournament is one of the toughest if not the toughest. Over 32 different national teams from different continent facing each other. Teams consisting of the best of the best player from each countries, they come not only to play football but to represent the dignity and integrity of millions of people in their country. The pressure, the expectations, the momentum is really mind blowing.

• Scarcity
  What I mean by scarcity is that it is only played once every 4 years, so a player can only play maybe maximum of 4 or 5 times in their career, this makes it really hard or impossible for every star player to win it. For example Cristiano Ronaldo and Messi despite being outstanding in club victorys and achievement they haven't been able to win the world cup, and now it's obvious this world cup would be their last. So that means either one of them wins or both of them won't win a world cup in their career.

With this few points i hope I have been able to convince and not to confuse you that the world cup is the most difficult cup to win.