Is the word best an adjective?

Yes, "best" is an adjective. It describes a noun, indicating the highest quality or degree.

For example:

* The best pizza in town. ("Best" describes "pizza")

* She is the best singer in the choir. ("Best" describes "singer")

However, it's important to note that "best" can also function as a superlative adjective, which means it compares something to all other things in its category.