It depends on your objective.. If you want to learn soccer and it principles I would recommend you to go to a country where Soccer is the first and most popular sport .. (Eg: Brazil, England).
If you are already an professional and want to advance in your career I would recommend you to go to any of the countries listed below by Jesse.
It all depends on where you live, where you want to live and where there are opportunities. The consensus top five leagues in the world are all in Europe (in no order):
- Premier League (England)
- La Liga (Spain)
- Serie A (Italy)
- Ligue 1 (France)
- Bundesliga (Germany)
England and Spain have the best soccer development schools in the world and they also expose majority of their players to the world for recruitment