I grew up believing (maybe I was even taught this) that fun, funner and funnest were the proper positive, comparative and superlative forms of fun.
But then I did some research and found some interesting debates going on:
It seems that fun began life as a noun and many people refuse to recognize it as an adjective. This is important because if fun is a noun and you want to use a comparative form, you write or say more fun; ditto with the superlative, you write or say most fun. To illustrate:
Playing cards with Joe is more fun, but playing cards with Jane and Joe is most fun.
Now there are those who insist that fun has morphed into an adjective, in which case my use of fun, funner, funnest would be correct. However, the most vocal out there (on the Internet) reject fun as an adjective.
But that’s because these people are no fun to begin with, and it’s funner to make the most fun out of them.