Ah, darn, Peachey beat me to it! Teacher knows best :)
I'll just try to rephrase it for you.
You can have "best" as an adverb, describing a verb: he runs best, she works best…
"Best" can also be an adjective, with a noun: he's the best runner, she's the best worker…
When using it as an adjective, when it's describing a noun, you should use the article "the". That is the most common function you are used to seeing it in and that's why you feel the need to add the article. Just consider what it's referring to and the answer is easy.