I would say probably Pokemon Diamond. I have like a 30 hour save and corrupted my card with a crappy rom trimmer. So I used cheats and got back to where I was and then played legit after that. I ended up playing the second time like 80-90 hours probably.
Also
TWEWY, just beat it, have about 60 hours, going for bonus missions
Mario Kart DS, it was the only DS I had for a loooong time. i have sold other games here and there, but one I have held on to is MKDS. I love that game.
And yes, animal crossing. My freshman year in college I didn't do anything except play that game and go to class. Then I needed cash and sold the game off.