Can tends to be used for: • Ability (I can ride a bike) • Possibility (It can still happen) • Permission (You can go now) • Informal requests (Can I borrow a pen?) Could tends to be used for: • Past tense of 'can' (I could fit into this when I was a kid, but now I can't) • Polite requests (Could you please move your car?) • Possibility (It could be she missed the bus)

We use can and could for lots of different reasons, ability, requests, suggestions etc. In the song can and could is used as part of a conditional sentence. The full sentence could be You could be the greatest, if you keep working hard. Can has more or less the same ability, talking about how this person has the ability to be the best. In this example, both can and could are talking about the future. http://www.pearsonlongman.com/ae/azar/grammar_ex/message_board/archive/articles/00096.htm