There's tons to pick from but I gotta give the gold medal to Triple Triad, for being not only widespread and enjoyable, but ridiculously useful in multiple fronts. Of course, that's part of its weakness, too: it's effects are so far-reaching that it can siphon away what little difficulty the game has left. Getting Degenerator two hours in is pretty funny, though...

Runner-up is probably Chocobo Hot & Cold. Digging up items isn't that awesome itself; it's using the world for a massive game of hide-and-seek, like a primitive geocaching expedition. And the flamenco music...way too awesome! Shame it's not heard outside of the minigame.