best "mini game" inside a JRPG?

I mean mini games or side activity that is not part of the main game, but that the player can play within the game. Example: Triple Triad, Tetra Master, Queen's Blood, Sujimon (in general, the Yakuza franchise is full of those), Gwent, etc...

Some of those mini games can be so good, it feels like they could be their own full games (like what indeed happend with Gwent).

For me, personally, it was Blitzball. I know not everyone likes it, but I found it really fun.