Though I'm not an expert,
1. He likes you the best.
2. I had the worst headache last night.
in "1." the best is describing you. And since best is a adjective describing you, THE can therefore act as a adverb describing best. so to summarize, THE describes adjective best and best describes object you
in "2." same idea as "1." the sentence itself isI had headache. But with adjective worst describing the object headache, and THE describing worst, THE therefore act as adverb
please correct me if i'm wrong
Student or Learner