Meruem returns but now he is fighting a DBZ character of incredible power and speed. While the two may have similar designs, Cell is a fighter who could end the entire solar system so that ends the comparisons between them. Technically speaking their personalities are also rather different so it’s really the design that gets brought up a lot. The power scales in DBZ and HXH are too far apart for Meruem to stand a chance though. Cell wins.
Meruem and Oga are both kings and as such have to try to rule in a way that would best serve their kingdoms and ideals. Oga is quick to abandon those ideals of his though which is why he will never be a match for Meruem. Meruem also has a great deal more power which is another thing to consider. It would be difficult for Oga to react to Meruem’s attacks, let alone counter them. Meruem wins.