Edit: The "math game" in FFX-2, I loved that one as well.
I forgot about this. It was amazing.
Monkey Billiards 2 from Super Monkey Ball 2. I'd say 99% of the time I fire that game up, it was to play pool.
Just listen to this music: https://www.youtube.com/watch?v=l14iNKitF0Y
I dunno if I'd go that far, as I beat the fuck out of that game (all platinum medals in the challenges and beat the game on whatever the hardest difficulty was (it was really fucking hard)), but I also beat all the high scores in the three minigames, so clearly I enjoyed them quite a bit as well. Anaconda was definitely the best, probably my favorite snakelike ever, but Astro Lander was a close second, and a really good Lunar Lander clone. Plus, dat music. (The third minigame was a pretty shitty topdown racer, unfortunately.)
I had more fun with Anaconda than I did TimeSplitters 2 itself.
Nothing's coming off the top of my head right now, but holy, if Mario 64 DS counts then that most definitely. Had to have spent 40+ hours in that thing.
The Mario 64 DS mini games.
They were addicting.
Edit: if you don't mean separate mode. I guess the card game in Digimon World 3