Mereum is considered to be strongest character in Hunter X Hunter by most people. However, there are certain characters who could possibly be stronger than Mereum and might win in a fight with him.
1. Maha Zoldyck
Maha Zoldyck is supposed to be a really strong fighter. Netero in his prime was known to fight Maha Zoldyck and Netero walked away from that fight. Supposedly, Netero was loosing the fight and decided to run away. So, Maha Zoldyck is probably going to be a very tough competitor even for someone like Mereum.
2. Zigg Zoldyck
Zigg Zoldyck is another character that might be really strong. Zigg went with Netero on an expedition to the Dark Continent. Zigg is also speculated to be very strong and he might have what it takes to beat Mereum.
3. Don Freecss
Don Freecss is a character that has been known to visit the Dark Continent over 300 years ago. He wrote ‘Journey to the New World’, a traveller’s guide to the Dark Continent which has two volumes ‘East’ and ‘West’. The East edition is with V6 while there is no sign of the West edition. It is speculated that Don is still writing it meaning that he is still alive. Considering being alive on the Dark Continent for over 300 years, Don must be quite strong if he is still alive.
4. Beyond Netero
Beyond Netero is the son of Netero. Considering his young age, he could possibly be significantly stronger than Netero. Netero in his prime might have beaten Mereum. Considering that, Beyond Netero may actually win against Mereum in a fight.
Alright, so this is the last character and Nanika might actually beat Mereum if she can use her powers directly instead of placing conditions and requests.
So, this is the list of characters I believe could possibly be stronger than Mereum.