N+. Awesome game, tons of value.
Tingle's Rosy Rupeeland. Not even kidding, either.
Both Professor Layton games. Tons and tons of playtime too.
The other Advance Wars DS game, dunno which one you have, but there are 2.
Mario & Luigi Partners in Time. If you like Bowser's Inside Story, this is the game that came before it.
Animal Crossing. Literally endless replay value.
FF IV. It's infinitely better than III.
Kirby Canvas Curse, Squeak Squad, and Superstar Ultra. 3 Awesome Kirby games.
The Castlevania games. Gems.
Hotel Dusk. Seriously, check this one out, it's unique and brilliant.
Mario Kart, best portable racer ever.
Elite Beat Agents. Hilariously addictive and lots of replay.
The Ace Attorney games are no-brainers. You must play them. 
... Those are off the top of my head, but there are many more.
Nintendo Network ID: Cheebee 3DS Code: 2320 - 6113 - 9046
