Most fun. See Usage Note at fun.
- (humorous, nonstandard) superlative of fun
- Funnest is a regular superlative of the adjective fun. However, the use of fun as an adjective is itself still often seen as informal or casual and to be avoided in formal writing, and this would apply equally to the superlative form. Merriam-Webster, however, gives fun as an adjective without comment, and states that funner and funnest are ‘sometimes’ used. Because of the remaining stigma, most fun is preferred in formal writing.