The best league by overall quality. The very best teams are better in Spain but other than that the English league is better. A team finishing 8th in the English league being favourites for a European tournament with the 4th and 5th team in the Spanish league being an example of that.
Leicester is a bad example, it was one of those seasons. They don't have the best players in the league, nobody would think that, how many players from Leicester would get into Chelsea's squad, a team finishing 10th? Not that many, maybe Mahrez and Kante. There are players in a number of Premier League teams that would get into such teams. De Gea, Aguero, Lloris, De Bruyne, Hazard, Coutinho, Kompany and Sanchez to name a few would get into at least one of those sides and many more.