Desert island scenario; if I had to pick just 3 DS games to be stranded with...
Advance Wars: Days of Ruin (Nintendo / Intelligent Systems) Great amount of content, well balanced campaign/COs, online play, map editor... really just the best package all around, even among the AW games imo. Literally infinite replayability.
Retro Game Challenge (XSEED / Namco / indieszero) So charming it hurts. And honestly, almost all the "classic" games at top notch. Really a love letter to those of us who grew up on NES ad SNES.
Mario Kart DS (Nintendo) Best racer on DS, online play, tons of courses, perfected handling and mechanics. Really, really great game.
...if you're looking at DSiWare, I'd strongly recommend Starship Defense, Trajectile, Art Style: PiCTOBiTS, Art Style: boxlife and Dark Void Zero. I think they all like $5 each, and they're all insanely deep!
