Hmm...for me, it would probably be a game like Radiant Historia. I don't have a DS anymore, but everything I've heard about Radiant Historia at times makes me wish I still had it. Maybe that'll get ported over to PSP or something so I can try that out.
When I owned a DS, probably my must-have game would have been Castlevania: Dawn of Sorrow or CV: Portrait of Ruin, depending on my mood.