fun 用作形容词的比较级和最高级是 funner 和 funnest。请看《新牛津英汉双解大词典》的有关说明：

The use of “fun” as an adjective meaning “enjoyable”, as in we had a fun evening is not fully accepted in standard English and should only be used in informal contexts. There are signs that this situation is changing, though, given the recent appearance in US English of comparative and superlative forms “funner” and “funnest”, formed as if “fun” were a normal adjective.