Intelligence is not the single best predictor of success. Intelligence is a form of natural talent. It relates to one's ability to function cognitively in the domains of pattern recognition, memory, imagination, recall, and abstract reasoning. Cognitive psychologists understand intelligence as the ability to perform these particular functions well.
The strongest predictors of success are the habits of delayed gratification and deliberate practice. Delayed gratification is the ability to put off a pleasurable experience in exchange for a more important gain. The famous Stanford Marshmallow Experiment demonstrated a strong link between toddlers' ability to delay gratification, and success across a number of indicators including educational attainment and income. People who, as toddlers, chose to wait a longer period of time for two marshmallows, did better in life than those who, as toddlers, chose the instant gratification of having one marshmallow now rather than wait for two marshmallows later.
Deliberate practice means working at the edge of one's ability in order to extend one's ability a little further. It is fully engaged, intentional, intense cognitive work. Cognitive psychologists have identified deliberate practice as a strong success indicator in chess, music, and athletics.
The two habits of delayed gratification and deliberate practice are better success indicators than intelligence (or any other form of natural talent).