The premier league is the most commercial and popular league. That's why it's easy to say it's the best, as part of the advertisement.
It's difficult to say which league is the best. Looking only at the top teams you could say Spain is the best. Look at the bottom and you would get the same garbage in germany, Spain or england. The difference between these leagues, probably the 3 top leagues, is that in england the bottom teams have the funds to buy new players, which gets fans and media all excited. In germany the clubs are more sensible with their money, so bottom teams rarely spend more than 5m on a player. In Spain, the smaller clubs are close to bankrupcy so they have to sell players and attract others for free.
So I don't think the premier league is the best when it comes to the level of football being played, but is the best in the sense that teams have the cash to significantly improve their team over a transfer period and a new season can realy be different to the previous.