In a strong-form efficient market no information of any kind, public or private, is useful in beating the market. Notice that if a market is strong-form efficient, it is necessarily weak- and semistrong-form efficient as well. Ignoring the issue of legality, it is clear that nonpublic inside information of many types would enable you to earn essentially unlimited returns, so this case is not particularly interesting. Instead the debate focuses on the first two forms.