I think the EPL and La Liga are clearly above the rest, but each is superior in some elements to the other in how you determine what is the "best league". I think that one is an endless debate. The metrics don't converge and an eye test will show players being exposed (and your run of the mill league player being incredibly impressive) in both leagues.
In terms of De Ligt, he is the most promising CB prospect out there, but whether he is worth the cost who knows. (Lucas Hernandez is a couple years older but has cut his teeth in national competitions and with Atletico, with a 90 mil buyout.) I also would severely doubt De Ligt is is ready to be a good consistent player in La Liga or EPL his 1st year. (For instance Daley Blind looks like an excellent player when playing in the Eredivisie) I will be interested to seem him play Real, while their attack has been in shambles this year they still have some pacy and skilled players they have not much to play for this year other than the CL.