Probably Harry simply because he has the biggest solo fanbase right now but in terms of talent it should be Zayn. Zayn isn't as accessible as Harry is so that probably turns some people off.
tbh I think 1D will be like the Backstreet Boys where none of them will really have a successful solo career. None of the members really stand out to me the way JT or Beyonce did in their respective groups.
If anything, it'd be Zayn because he looks 10x better and his vocals are 10x better but with the boring personality he has I don't think it could happen. It def wouldn't happen with Harry.