Yeah, I've been reading up on this since you brought it up. It's been more fun than a barrel of monkeys. Or has it been funner than a barrel of monkeys?
(Both terms are used on the Internet.)
It seems that fun used to be only a noun or a verb. Times have changed, though, and fun is now also considered an adjective (but not without some kind of labeling, like
informal, in many of the dictionaries I checked).
When used as an adjective, English allows two different ways to form the comparative and two different ways to form the superlative.
For example, both sets are equal:
hot : more hot, most hot
hot : hotter, hottest
So, it stands to reason that both of these sets are equal:
fun : more fun, most fun
fun : funner, funnest
Based on Internet usage alone, more fun appears to be more popular (popularer

).