The comparative and superlative forms of "fun" can be a little tricky because "fun" traditionally acts as a noun, but it’s often used as an adjective informally. Here’s how it works: More fun and most fun are the standard forms, as in: "This game is more fun than the last one." "This is the most fun I’ve had in ages." "Funner" and "funnest" are also used informally, but they’re less common and not generally considered standard in formal writing: "That ride was even funner than the last one!" "This is the funnest party I’ve been to!" For formal contexts, it’s best to stick with "more fun" and "most fun," but "funner" and "funnest" are widely understood in casual speech.

more fun / most fun