Judgement or fact
Relative clauses normally take the indicative. After certain antecedents, though, the subjunctive can appear: after a superlative, after le seul, l'unique, le premier, le dernier, after negated quantities like personne, rien, aucun, and with things you're looking for whose existence is uncertain. The underlying question is always the same: are you stating a verifiable fact, or giving your own assessment?
The pattern
After superlatives and le seul
After a superlative or le seul, the subjunctive is the careful choice when you're evaluating. When you describe a verifiable fact, such as a record or a date, the indicative is usual.
Evaluation or fact
Subjunctive, judgement
C'est le meilleur film que j'aie vu cette année.
It's the best film I've seen this year.
C'est la seule collègue qui sache vraiment utiliser ce logiciel.
She's the only colleague who really knows how to use this software.
Indicative, fact
Armstrong est le premier homme qui a marché sur la Lune.
Armstrong is the first man who walked on the moon.
C'est la seule candidate qui a répondu à temps.
She's the only applicant who replied in time.
After negation and open searches
When you deny that something exists (personne qui, rien qui, aucun … qui), the relative clause takes the subjunctive. The same goes for something you're looking for or asking about when it's unclear whether it exists. Once you've found it, the indicative returns.
Il n'y a personne qui puisse la remplacer.
There's nobody who could replace her.
Je ne connais aucun restaurant qui soit ouvert le lundi.
I don't know any restaurant that's open on Mondays.
Il n'y a rien qui me fasse plus plaisir.
Nothing makes me happier.
Vous connaissez quelqu'un qui puisse m'aider ?
Do you know anyone who could help me?
Sought or found
Sought, subjunctive
Je cherche un appartement qui ait un balcon.
I'm looking for a flat with a balcony.
Found, indicative
J'ai trouvé un appartement qui a un balcon.
I've found a flat with a balcony.
