While "funner" and "funnest" are listed in many dictionaries and appear in news articles, they are sometimes considered to be grammatically incorrect. In practice, the words "more fun" and "most fun" are much more widespread in usage and are considered the correct forms of comparison.

More fun is accepted; funner is not. Most fun is accepted; funnest is not. If you want to be taken seriously as a writer or you are an ESL student learning the language, do not use funner or funnest or your writing. Fun is increasingly being used as an attributive adjective in informal contexts.

The move to adjective has gone so far in the USA that its comparative and superlative are not infrequently found in informal writing: funner for ‘more fun’ and funnest for ‘most fun’ (“basketball is funner than football, and soccer is the funnest game of all”).