I can't really name a single one that would be THE one. But the one I returned to the most over the years was the original Dragon Quest Monsters.
I had it on my Game Boy since I was in elementary school and even then played through it multiple times. Later I played through it multiple times on emulators. I just love this game so much.
It has a lot of flaws, starting with the limitations of a Game Boy game, but later games and other monster collectors never captured my heart like this one did.