Its a tie between Rune Factory (The first one), and Digimon World DS (All of them, the RPG ones, and the Championship one).
I got Rune Factory during a vacation to my grandparents (My dad knows its boring as crap for me, so its usually when i get a bunch of free games, and i decided to grab it), i literally played it non stop for the next week until i beat it (It even made me put down Luminous Arc, which i had practically speed run'd through in the first 3 days). And Digimon World, was just so addictive since im an old school digimon fan, plus it had Wi-Fi.