Cases where certain plural nouns always take definite article?

Hi. Please help. I think we almost always use the definite article "the" before a plural noun that has certain adjectives like "first" or "best" before it eventhough a sentence may be general in meaning. But unlike sentences with those adjectives like "first" and "best," certain adjectives like "optimal" which means similar to one of those adjectives may not have the definite article before a plural noun even though a sentence may be general in meaning. Is that correct? Thank you in advance.

eg,
They have the best teachers in their institutions.
The best teachers are hard to recruit (get).
They implemented optimal measures to tackle the problem.