Yoshi Touch & Go: A hidden gem of a near-launch tech demo game. Short, sweet, arcadey, and perfectly shows off the DS hardware.
WarioWare D.I.Y.: A modern Mario Paint where you can make your own microgames, music, and (barebones) comics. You can still download other players’ creations today through unofficial servers, or transfer them to DIY Showcase on the Wii to play them on a big screen.
Tetris DS and
Art Style PiCTOBiTS: These are both highly recommended for fans of the NES. Tetris DS has a good variety of modes, and PiCTOBiTS is an action-puzzle love letter to the NES. Just listen to this music:
Mario Party DS: One of the best Mario Parties to this day, maybe even up there with the N64 games and Superstars. Don’t try to unlock everything, you’ll be playing forever.
Pokemon Mystery Dungeon: Explorers of Sky: A cult classic, and definitely not obscure, but you’re missing out if you haven’t played it. The Project Orion rom hack adds a lot of QoL improvements, but also changes the list of possible starters (not better or worse, just different) and removes the personality quiz. If the starters bother you that much, you can open the rom in SkyTemple and change them.