Hello damita.jo, superlative adjectives generally need the definite article, and that is the case in your example:
The best teachers are competent and creative. ("Best teachers" without the article is not possible ihere.)
However, it appears that when one object is being compared with itself, the article is not used, according to this explanation (source: aclil2climb.blogspot.co.uk). I'll reproduce one example ifrom that page in case the link gets broken at some future stage: "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)."
Applying this explanation to your example, you get "teachers are best when they are competent and creative."(Not "the best")
And note the last comment on the linked page: "Generally speaking, we don't use the definite article with superlatives if they are preceded by a possessive adjective or a Saxon genitive.
E.g: Susan's eldest son is a computer wizard.
Not: Susan's the eldest son..."
Why is that? Simple! Possessive adjectives and the Saxon genitive implicitly contain the definite article already. When you say "my", you say "the my" . Similarly, when you say "John's car", you say "the John's car" . This is a grammar point that is sometimes overlooked.