That was the worst thing you could have said, sweetling. Source: Game of Thrones Hi. I think the "could have" here expresses past options. It implies that at the time you could have said other things instead but you did't. We can replace "could have said" with "could say" without changing the meaning, but the implication will be lost. And I think all the uses of "superlative+could have done" can be interpreted this way. Am I right? Thank you.