Some English traditionalists claim that the only correct comparative form of the adjective fun is more fun, that the only superlative is most fun, and that funner and funnest are only appropriate in the most informal contexts. This rule might once have been justifiable, but today it is obsolete, and it lives on only because not enough people have dared break it. This is beginning to change, as the one-word forms have gained ground in recent decades and have even worked their way into edited writing, but there is still a long way to go.
For now, the fact remains that funner and funnest are, erroneously or not, still considered unacceptable by many editors and other careful English users. So if you are writing, say, a school paper or a job application letter, you might play it safe by using more fun and most fun. The bias against funner and funnest runs deep, so we’re probably going to have to contend with it for another generation at least. Useless language proscriptions tend to long outstay their welcome.
The reason the use of funner and funnest has been discouraged is that fun was until recently only a noun. Nouns do not have comparative (-er) and superlative (-est) forms, but mass nouns such as fun can be modified by more and most (e.g., “I have more water,” or “he has the most courage”). But while some of the stodgier English reference books still pretend fun is not an adjective, most English speakers moved on long ago, and the adjectival fun is rarely questioned. Ultimately, if we accept that fun is an adjective—and we have no choice, because it’s common—then we also have to accept funner and funnest. Comparatives and superlatives of one-syllable adjectives usually take the -er and -est endings, and there’s no good reason fun should be any different.
The bias against funner and funnest is still strong in edited publications from throughout the English-speaking, but it is possible to find some exceptions:
Apple once again getting ahead of the game, offering something cuter and funner and more Appley than anyone else. [New York Times]
Which is why it’s hardly a surprise that parties – the funnest of all the supposedly fun things – ain’t so fun without booze. [Telegraph]
The RNC should have been the funnest place on Earth to be a Republican this past August 27-30. [Appeal Democrat]
[It] is funner if you don’t have next week’s grocery budget wagered on the over/under. [Portland Daily Sun]
One of the funnest things about playing with data is that you can clash ideas up against one another. [The Age]
Let’s be honest, the veepstakes is the funnest part of any presidential election. [New York Magazine]