My top 5 DS games.
1. New Super Mario Bros-2D mario game, enough said.
2. Castlevania- Dawn of Sorrow- arguably the best of the Metroidvania games
3. Mario Kart DS- arguably the best in the Mario Kart series, though I ditched it a while back as I was sick of it
4. Kirby Canvas Curse- the only semi-traditional game I've played that made good use of the touch screen
5. Planet Puzzle League- tetris attack is still my favorite puzzler, and this is the latest version with a lot of modes, online pay etc.
Some others worth mentioning: Castlevania-Portrait of Ruin, Puzzle Quest, Picross, Brain Age 1 or 2 (mainly for Suduko for me), Kirby Super Star Ultra.
There are also a lot of good JRPGs and text adventures if those are your cup of tea. They're not mine and it's led to me not buying many DS games the past couple of years. You'll notice the vast majority of what I listed above came out in 2006 or earlier. Seems like most games the past couple of years have been JRPGs, strategy rpgs, text adventures or licensed shovelware. Still a great little system, but it's been slim picken's for me lately. Did just buy Kirby Superstar and looking forward to the new Castlevania.