Hello everyone,
I was wondering which of these pieces is the most difficult : Mephisto Waltz or El Contrabandista ? Thank you ! 
As I previously commented, you ought to consider creating your own showy arrangement of something like "Despacito" instead of studying El Contrabandista. The point of it was to titillate Spanish audiences with songs they knew so you're guaranteed to have some popular appeal the same way if you did that.