I will share a bit of my DS insider knowledge...
1. Bangai-O Spirits (DS):
In my opinion the best DS game. Yes - the very BEST. And nobody seems to have heared of it. The level editor makes it perfect.
2. The World Ends with You (DS)
Not that underrated, but still pretty unknow. The second best DS game, along with Pokémon.
3. 999: Nine Hours Nine Persons Nine Doors (DS)
Well, ok. It sold out. But it is still pretty unknow, at least in Europe (it didn't get an EU release). A visual novel with an intriguing mysterious story.
"Nine people have been abducted by a mysterious kidnapper who uses the alias "Zero." They find themselves on a ship— possibly a replica of the RMS Titanic and are told that they have nine hours to escape before the ship sinks beneath the waves. Zero is running the "Nonary Game" - a game "where you will put your life on the line." The group is forced to split up into various subgroups and explore "numbered doors," behind which lie Zero's puzzles. Zero promises that escape lies behind a door numbered [9]. The characters must work together despite suspicions of each other to advance, as well as to discover Zero's motive and identity." - Wikipedia
4. Big Bang Mini (DS)
This game is so beautiful. If you like arcade even a little bit, this is a must get! Please watch
this review.
5. N+ (DS)
Not the very best DS game, but definitely great and worth getting. A really hard platformer. Also features a level editor!
Dragon Quest =/= Dragon Quest Monsters! Monsters is a spin-off!