There's a lot of "must play" games on the DS, it's probably the best software library after the SNES library, but here are my top 3 preferences:
1) Dragon Quest V - The Best DQ game ever made and my personal favourite video game story ever. I had played it before (rom emulation with translation patch), but this version is much more polished with some good bonus content added to it.
2) Pokemon Black&White - The best Pokemon games after the original Gold/Silver/Crystal from the GBC. The new battle look is pretty stylish and the new battle mechanics are also very good additions to the series. Plus it's the first pokemon game to have some sort of plot.
3) Dragon Quest IX - Not the best DQ game out there, mainly to the lack of an actual good plot in detriment of a massive focus on gameplay. The game itself is pretty massive and will last you over 200 hours, at least. The most complete DQ game out there and one of the best RPG's of this generation.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
