top 5, as follows:
1. Rocket Slime
2. The world ends with you
3. Pokemon diamond
4. Advance wars: dual strike
5. Legend of Zelda: phantom hourglass

Out of what i've played on the DS...

Metroid Prime Hunters
Castlevania: Portrait of Ruin
Mario Kart DS
Elite Beat Agents
Final Fantasy XII: Revenant Wings

1. New Super Mario Bros.
2. Touchmaster
3. Little Red Riding Hood's Zombie BBQ
4. Tetris DS
5. Geometry Wars

(note: memories may be skewed)

Ace Attorney series
Chou Soujuu Mecha MG (Super Control Robot MG, Japan only)
Castlevania series (though I think I liked the GBA ones more, DoS was the best of the DS ones, PoR was kinda meh and I'm not sure what to think of OoE, didn't care much for the increased difficulty though the glyph system was nice)
Zelda Phantom Hourglass (2D Zelda > 3D and the temple wasn't much of an issue if you actually made useful notes about the things you found and applied new items properly, changing the way it played every time)
Megaman ZX series (I just wish they'd finally ditch the boss refights because fighting 8 bosses in a row and then the hardest boss in the game with 3 phases on one set of lives is plain frustrating and doesn't really make me want to finish the game)

It's a tough question, but my favorite DS games are:

• Professor Layton and the Curious Village
• Densetsu no Starfi 4
• Rhythm Heaven
• Advance Wars: Days of Ruin
• Fire Emblem: Shadow Dragon
• Hotel Dusk: Room 215
• Pokemon Platinum
• Kirby Super Star Ultra
• ClubHouse Games

Oddly enough, they're all published by Nintendo.

Mario Kart DS
Elite Beat Agents
New Super Mario Bros
Puzzle League DS
The Legend of Zelda Phantom Hourglass

I also like Picross and Geometry Wars Galaxies - but not quite enough to make it to the top five. Once I play through a bit more GTA, I may need to add that one to the list as well.