funnest

Adjective

 * 1) * 2007, Terry Francona, Pressure on Red Sox as ALCS shifts to Cleveland
 * That was one of the funnest games I've ever been a part of until the very end, and then it rapidly became a whole lot not of fun. We lost in kind of an ugly fashion at the end.
 * That was one of the funnest games I've ever been a part of until the very end, and then it rapidly became a whole lot not of fun. We lost in kind of an ugly fashion at the end.

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