I suppose it depends on the criteria but in my view none of the 7 on this list (I have not had the pleasure of N'djamena, Beauvais, or Tribhuvan) that I've visited come close to qualifying as world's worst, although Guaralhos is an experience I don't wish to repeat.
For a country that calls itself civilised, CDG has to be the worst experience, dirty, smelly, inadequate facilities, inefficient and diabolically rude staff. Madrid Barajas comes close in terms of the staff and facilities. Berlin Schoenefeld must be an embarrassment to Germany.
MIA is awful, disjointed and over capacity, but I think the worst I've ever been to is Murtala Mohamed in Lagos, however it reflects the city it serves!