As is mandatory on all good DS RPG threads: Chrono Trigger.
Now for the real list:
Final Fantasy IV
Final Fantasy Tactics: A2 Grimoire of the Rift
Disgaea DS
Chrono Trigger
Dragon Quest V
Megami Ibunroku(Shin Megami Tensei): Devil Survivor
Avalon Code
Kingdom Hearts 358/2 Days
Phantasy Star 0
Final Fantasy Crystal Chronicles: Echoes of Time
The World Ends With You
Sands of Destruction
Suikoden Tierkreis
I'll post more later.