I love the 3DS to death. TO DEATH. But if you only really need one game - A Link Between Worlds. It kept me hooked and is one of the few games I did to 100%. It's just so damn rewarding and the side quests are excellent.
For a download game - SteamWorld Dig is much better than you'd think. It's really just about tunnelling and getting better equipment to dig deeper but again it kept me hooked for a week solid. It's another game where I wanted to see it all.