What part of speech for best?

"Best" can be a few different parts of speech, depending on how it's used in a sentence:

* Adjective: Describing a noun. Example: "That's the best cake I've ever had."

* Adverb: Modifying a verb, adjective, or another adverb. Example: "She sings best when she's happy."

* Superlative Adjective: Comparing three or more things. Example: "This is the best of the three options."

To determine the part of speech, you need to consider the context of the sentence.