I have the best partner in the world

Best in the world is a comparison nobody can actually run. What people mean by it is stranger and more precise: this one knows I take my tea weak, and remembers the anniversary of my father's death without being reminded.

Nobody else alive is keeping that particular file on you.

The superlative is really about fit, the accumulated and unrepeatable knowledge one person has of another. On the days your partner is irritating rather than best, the file is still open. Someone is still adding to it.