Past perfect with superlative forms of adjectives

The past perfect is used with superlative forms of adjectives to express emphasis:

It was the strangest piece of music I had ever heard.
Jill was the most beautiful girl John had ever met.

The second sentence above implies that until that time John had never met such a beautiful girl, but since then perhaps he has. If, however, John hasn't met anybody like Jill since then, the present perfect is used instead of the past perfect:

Jill was the most beautiful girl John has ever met.