The leader or superior, i.e. alpha male, of a group, clique, or social hierarchy. First banana is followed by the second in command, or second banana, who has reign over all bananas (e.g. third, second, fourth banana) excluding the first. Lower bananas are typically prone to the whims of upper bananas and can even become the subject of coercion or ridicule at the superior banana's expense.
Barack Obama is first banana; Joseph Biden is second banana.