adjective
A word that describes or modifies a noun or pronoun. In this case, 'funnest' is a superlative form of the adjective 'fun', indicating the most fun out of a group of things.
The amusement park advertised their newest ride as the funnest attraction in the park.
I had the funnest time at the concert last night!
That party was the funnest thing ever!
Learning a new language can be the funnest journey of self-discovery.