funner

Adjective

 * 1) * 1979, Juvenile Justice and Delinquency Prevention in Wyoming, United States. Congress. Senate. Committee on the Judiciary. Subcommittee to Investigate Juvenile Delinquency
 * "I wish I would have played basketball when I was out. It's a lot more funner."
 * 1) * 1992, Ana Malinow Rajkovic, Manual for (Relatively) Painless Medical Spanish
 * Sounds like a great vacation, doesn’t it? It would be much funner, undoubtedly, to spend four weeks learning intensive Spanish in Guadalajara... but what do you expect for this price?
 * 1) * 2000, Julia Bourland, The Go-Girl Guide
 * Every wardrobe needs an all-purpose cocktail dress, but these are often funner if you can find a retro party dress in flawless shape at a vintage clothing store.
 * Every wardrobe needs an all-purpose cocktail dress, but these are often funner if you can find a retro party dress in flawless shape at a vintage clothing store.

Usage notes
While funner is a regular comparative of the adjective fun, the comparative more fun is much more common. 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 comparative form. Merriam-Webster gives fun as an adjective without comment, and states that funner and funnest are "sometimes" used.