Since we're talking about the 20th anniversary of the #NintendoDS and people are listing their top five favorites, here are mine (I don't have pictures because I'm lazy and, uh, might have gone mostly 3DS some years ago):
Rune Factory 3
Etrian Odyssey II
Megaman ZX Advent
9 Hours 9 Persons 9 Doors
Shin Megami Tensei: Strange Journey