more fun BTW, "fun" is a noun

Fun, more fun, the most fun. 'Fun' can be an adjective and it means the same as 'enjoyable'. "It is more fun (or more enjoyable) to spend time with friends than to spend time alone."

most fun, less fun, more fun fun fun fun :P

I think you meant "funny". It's "funnier". "Fun" is a noun. More fun, less fun.

Oxford dictionary states that "fun" is a noun, a verb and an adjective. It even shows the forms of the adjective as fun, funner and funnest, although I must admit that I have never heard of funner or funnest. When fun is used as an adjective it must be followed by a noun, e.g., It was a fun activity e.g., It was a/the more fun activity. e.g., It was the most fun activity. And if anyone believes the Oxford dictionary: e.g., It was a fun activity e.g., It was the funner activity. e.g., It was the funnest activity. Wow - those just don't sound right to me, but who am I to argue with Oxford ...