I think you're reading objectiveness into the question. If it makes you feel better, replace "best" or "worst" with "most favorite" and "least favorite". The essence behind the question is still there, but it removes any interpretation of objectiveness.

However, I would argue that it also loses some impact. It removes the idea of a person having to decide what "best" or "worst" actually means. Instead, you would need a follow-up question of "why?" to clarify that particular point.

For instance, I would have answered French horn. That instrument can single-handedly cut through an entire orchestra, and there is no mistaking it when you hear it. The harpsichord would be a close runner-up. So I'm probably looking for unique sounds that are easy to pick out of a crowd.