Is there a nen user that could've defeated Meruem (the king)? During the Chimera Ant arc of HxH, we witness the awesome battle between Meruem or the chimera ant king, and 


   Isaac Netero.


At the end,


   Netero is defeated by the king's might (some may argue that Netero still won with the hidden bomb, but in essence and for this post, I am going to conclude that Meruem defeated Netero).  


If 


   Netero


could not defeat Meruem, is there a nen user in the HxH universe that could? Perhaps someone from the main cast, such as Gon, Killua, Kurapika, etc., a lesser character, such as Ging, Kite, Illumi, etc., or maybe a character that was only briefly mentioned and not fully developed?
Just from the point of the show, it is clear that the only character with a chance of beating Meruem in 1v1 is....


   Adult Gon Pitou admitted that the insane power up may even challenge Meruem and thus sacrificed herself.You should find this reddit pretty interesting. Reddit: Adult Gon vs Meruem


Other characters with a chance/broken abilities are:


Chrollo Lucifer: The leader of phantom troupe with a most broken ability. There is no limit to the power of Chrollo and hence he will be the first in this list. Given the right circumstances and planning, he is sure to make an impact.
Zeno Zoldyck: At least as powerful as Netero, the head of master assassins Zoldyck is surely VERY powerful and capable of standing up to Meruem. 


However once, Meruem absorbed Pouf and Youpi, I doubt anyone could have beaten him in the HxH universe.

From another answer, 


  When it comes to a fight between Nen users, pure power doesn't mean everything. One may have an ability than can overpower a type of opponent.


Other then the anime, there have been host of new characters recently shown for the dark continent arc that can have an impact. 


  There are indications Maha Zoldyck is more powerful than Zeno or Netero. He definitely was at some point but is Zeno's grandfather. Like many other characters we don't actually have enough data to say. – kaine 
  I didn't remember Maha from the show. Haven't read the manga before the HxH_2011, I couldn't honestly comment on it, since Maha does not appear during the Yorkshin arc in the 2011 anime. However Wikia makes an interesting comment on him and he may well defeat Meruem if he was on his peak
  
  
    Maha is the only known Enhancer in the Zoldyck family. In his youth he was incredibly powerful, to the point that Isaac Netero, once considered the strongest fighter in the world, received praise just for surviving a fight against him: he was, in fact, the only person that far to challenge Maha and live to tell the tale. It is unknown if his strength has waned with old age.