Very interesting discussion. If you just look at the titles, Fu Haifeng is definetely up there. And don´t get me wrong, he was incredibly good. Maybe the best back court player ever and good defense as well. But for me he lacks that certain kind of brillance, the other candidates have. He was basically a powerhouse.And that he won his titles with different partners should not be weighted that much. Simply because his partners were Cai Yun and Zhang Nan, who both are at least on Fu`s level. Actually I would consider both of them even stronger. Compare that to Kido, Ahsan, Bach or Yoo. These players were pretty good, but not as exceptional as their partners (except Ahsan on certain occasions). So Fu Haifeng definetely had an advantage with his partners.
And just looking at the big titles is not sufficient, because a little luck and good timing is alsways necessary. Just look at Setiawan, who could easily have more Olympic medals or WC titles. But he lost 3 years of his prime with an injured Kido, the Olympics 2012 among them. And Setiawan/Ahsan prime stopped 2015, a few months before Olypics in Rio. So don`t get me wrong, Fu`s 2 Gold and 1 Silver medals is amazing and he is up there. But skill wise, I rate Cay Yun, Gunawan and Setiawan definetely higher. Lee Yong Dae is a special case for me. He was such a strong allround player with flashes of brilliance. But his biggest strength was consistency. Of course he was good at the net, but I was never left in awe watching him, because it was always the same, without surprise elements.