Superlative adjectives without the definite article 'the'

We've always repeatedly told our students, 'Don't forget the definite article when using the superlative!' so it wasn't all that surprising when one of my students, Saulo, asked me the other day, 'Isn't that sentence wrong? Isn't it missing the definite article?'

I don't remember the exact sentence in question, but it isn't important. I'll explain the rule anyway.

When we are comparing between three or more objects, we normally use the superlative with the definite article:

New York is the most exciting city in the world. (We are comparing New York with many other cities).

Usain Bolt is the fastest man in the world. (We are comparing Usain Bolt with all the other men in the world).

My boss is the most generous person I know. (I'm comparing my boss with all the other people I know).

However, when we are comparing one object with itself, we do not use the definite article, 'the'. This is best illustrated with examples:

New York is most exciting in spring. (We are comparing New York with itself, albeit at different seasons, so we DON'T say New York is the most exciting in spring).

Usain Bolt is fastest when he is running with very fast runners. (We are comparing Usain with himself, not with other runners: we compare him when he's running with fast runners and when he's running with slower runners).

My boss is most generous when we make a big sale. (Again, I am comparing my boss at different times, so the definite article is not used).

Well, Saulo, I hope this little explanation has clarified your doubts!