I think it's completely down to recency, at least if you look at global interest. The Clasico is still the biggest game, simply because Barca and Real are still considered to be among the 4-5 best teams in the world and their trophy cabinets in the last 10 years or so supports that claim. However, you immediately see global interest in this match go down if the quality of the teams suffers, e.g. by losing Cristiano or Barca not being as good as they used to be.
So I'd say the second place belongs to Liverpool vs City and after that it is very hard to tell. Probably CL matches between two big teams, like PSG vs Barca or Juve vs Real.
It's quite disappointing that United isn't really competitive right now because with the quality of the current Liverpool side, this match would have the potential to rival the Clasico.