Ds to my mind has a huge and awesome library. The 3ds has a is smaller library but many of its games are great.
Here are a few of my most fondly remembered ds games:
All the ds Castlevainias
Chrono Trigger
Mario 64 ds
NSMB
Mario Kart ds
Might & Magic Clash of Heroes
Disgaea
Trackmainia
Advance wars (Both)
Fire Emblem
Devil Survivor
Spirit Tracks
Browsers Inside story
Ace Attorney
Lock's Quest
Dragon Quest IX
Fighting Fantasy: The Warlock of Firetop Mountain
Professor Layton and the curious village
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