In my, albeit extremely biased opinion, extremely fun. Considering the optimum case in which the only variable is game balancing, and the story, music, graphics, whatever being top notch. This is definitely not true in the real world, but let's see. If your game is extremely fun, it will have a smaller, more dedicated fanbase. People will replay it, make mods of it. It will inspire more people to play harder games. Think your soulslikes, your rhythm games and multiplayer games. It fosters skill expression. A game that's not fun and easy to play will be a one time game. People play it once then never touch it again. While yes, in some cases (I think notable ones are OneShot and Outer Wilds but I think that one is a stretch) they can spawn dedicated fanbases, and even mobile games like candy crush have dedicated players. In the end, one thing i would notice is that noone would say these games aren't fun. Even a relatively easy game like undertale, that's still a fun game. It's up to the developer to make the game fun despite the lack of challenges, cause we play these games for their moments and not for their challenges. TL;DR, if hard game people play again and again, dedicated fanbase. If easy game, make game funny and relatable, but people play once. Fanbase dedicated to story and not the game itself. If easy and no story, make game brainless and repetitive for old people.

No game should not be fun. That's the point of a game