As mentioned, it's far too subjective to narrow down a definitive answer. In the loosest sense of the term, any international player could be labelled world class. If you are one of the best 20 footballers in a nation of millions then you are world class. Obviously some nations have a larger pool of talent to draw from. Still if you are a player who's appeared in a WC then you could rightly consider yourself World Class in a sense. Seperating the Class from the World Class is where it gets difficult. One could argue that results should be the official measure, though that shouldn't necessarily be the only consideration.