After a superlative, you choose
After a superlative (el mejor, lo más bonito) and after el único, el primero, el último, both moods are correct in the relative clause. The indicative is the default: you report what you have experienced. The subjunctive generalises: you compare with everything that could possibly count. It sounds more emphatic and slightly more elevated, often with nunca or jamás, much like English ever.
Same statement, bigger claim
Indicative
Es la mejor película que he visto.
It's the best film I've seen.
report
Eres la única persona que me entiende.
You're the only person who understands me.
statement of fact
Subjunctive
Es la mejor película que haya visto nunca.
It's the best film I have ever seen.
verdict, emphatic
Eres la única persona que me entienda.
You're the only person who could ever understand me.
rules out everyone else
Es lo más bonito que me hayan regalado jamás.
It's the loveliest present anyone has ever given me.
Es el último libro que escribió antes de morir.
It's the last book he wrote before he died.
plain fact, so indicative
Negation that rejects a claim
A negation on its own does not trigger the subjunctive. What counts is what you negate. If you deny that something exists or that a reason applies, use the subjunctive. If you are talking about something that does exist, the indicative stays.
No conozco a la persona que lo hizo.
I don't know the person who did it.
The person exists.
No conozco a nadie que lo haya hecho.
I don't know anyone who has ever done that.
No such person exists.
No es que + subjunctive
You reject an explanation that is in the air, and often supply the real one straight away.
No es que no quiera, es que no puedo.
It's not that I don't want to, I just can't.
No es que sea caro, pero no lo necesito.
Not that it's expensive, but I don't need it.
No porque + subjunctive, sino porque + indicative
The rejected reason takes the subjunctive, the real reason the indicative.
Lo dejó no porque fuera difícil, sino porque se aburría.
He gave it up, not because it was hard, but because he was bored.
No digo que + subjunctive
With a negated decir you explicitly avoid making a claim.
No digo que sea fácil, digo que es posible.
I'm not saying it's easy, I'm saying it's possible.
