Taken from Wiktionary : 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 may be preferred in formal writing.
english.stackexchange.com/users/1712/PatrickS
Given that every English speaker would understand you, it's fine.
It's a word.
Don't worry too much about what dictionaries say, they are trying to report on the language, not prescribe it.
english.stackexchange.com/users/133/Joel Spolsky
"Funnest" has good English morphology.
For instance, it doesn't begin with the glottal "ng" sound, which doesn't occur in English at the start of a word.
That kind of thing would tend to spoil all chances of an utterance from becoming an English word.
It is derived from "fun", which is already an English adjective, and it is derived according to a rule that derives many existing superlatives.
So you have here a good candidate for a word which could easily pass into usage among a group of English-speaking people, and evidently it has.
The only problem is that it's not widely used.
The human brain probably handles language not simply with with rigid rules, but through the memorization of a large number of special cases and exceptions based on examples and correction during language acquisition.
This is why numerous irregularities can happily exist.
Examples that are not heard are sometimes ruled out by native speakers as ungrammatical.
"Funnest" sounds strange simply because it's rarely heard, other than from children (who are usually going to be corrected to say "most fun" instead, thereby ensuring that they too will come to regard "funnest" as strange, when they will have long forgotten that they uttered it when they were small.)
On the other hand, "funner" and "funnest" are not so unheard of that it has not made its way into some dictionaries, such as Merriam-Webster.
If a word is in at least one major dictionary, it's safe to regard it as a word.
Some dictionaries may be more conservative than others, but, on the absolute scale, they are maintained by some pretty darn conservative people who will not too casually lend their support to a new word.
If some of them think something is a word, that's probably good enough for the rest of us.
english.stackexchange.com/users/19579/Kaz
I wouldn't use "funnest" anywhere but in the most colloquial setting.
Certainly I would not use it in anything written.
english.stackexchange.com/users/1731/Andy Lester
As reported by the NOAD, fun is used as adjective in informal contexts; its superlative is funnest , while funner is its comparative.
english.stackexchange.com/users/252/kiamlaluno
Funner makes me shudder.
For some reason "more fun" is firmly embedded in my psyche as the correct expression.
"funnest" doesn't raise the same reaction, for reasons I don't understand.
When someone says "it's funner", it makes me believe that there's been a lapse in their education.
english.stackexchange.com/users/8319/David Luebbert
We seem to be stuck at an impasse on this issue.  Is  funnest  a word or not?  If so, does it mean "most fun"?
Is "funnest" a word?
english.stackexchange.com
<meaning><grammaticality><adjectives><superlative-degree><is-it-a-word>
english.stackexchange.com/questions/4066
english.stackexchange.com/users/1704/tzenes
It is generally agreed that "funnest" is an acceptable word and that it is the superlative of the adjective "fun". Given that every English speaker would understand you, it's fine. There is a divergence of opinion as to whether it is a term to use or not. Some people feel it is a word every English speaker would understand and its absence in dictionaries should not be of concern, it suffices it appears in one major dictionary because ssme dictionaries tend to more conservative than others, but generally dictionaries are written by conservative people not inclined to support newly coined words. It is the superlative form of "fun". It sounds strange because it is not often used, except by children who are often corrected to use "most fun" instead. It should only be used in colloquial settings and not in formal language. Funner is the comparative form and "funnest" is the superlative. To some people these are strange form and the use of "it's funner" to some people suggests the speaker has a low level of education.
It is generally agreed that "funnest" is an acceptable word and that it is the superlative of the adjective "fun". Given that every English speaker would understand you, it's fine. Strictly speaking,  fun  is used as adjective in informal contexts; its superlative is  funnest , while  funner  is its comparative. 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. The main problem is that it's not widely used.  "Funnest" sounds strange simply because it's rarely heard, other than from children (who are usually going to be corrected to say "most fun" instead.)  "More fun" is what most people would regard as the correct expression.
Funnest is an informal word, and is  is a regular superlative of the adjective  fun . It is generally agreed that "funnest" is an acceptable word and that it is the superlative of "fun".
It is the superlative form of "fun" and is generally used informally. However, in formal writing "most fun" is preferred. It is a word every English speaker would understand. The absence of the word in dictionaries should not be of concern, it suffices it appears in one major dictionary. Some dictionaries tend to more conservative than others, but generally dictionaries are written by conservative people not inclined to support newly coined words. It is the superlatie form of "fun". It sounds strange because it is not often used, except by children who are often corrected to use "most fun" instead. 
Strictly speaking,  fun  is used as adjective in informal contexts; its superlative is  funnest , while  funner  is its comparative. 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. The main problem is that it's not widely used.  "Funnest" sounds strange simply because it's rarely heard, other than from children (who are usually going to be corrected to say "most fun" instead.)   "More fun" is what most people would regard as the correct expression.
"Funnest" is the superlative form of "fun". It is agreed that this is an acceptable word.