Ok, I want to let my fanboysm down and not mention Jump Ultimate Stars game in almost all the points, so here we go:
DS, It was gifted for me from a sister, I had a GBA advance at the time, and the touchscreen is what it sold me, I don't remember any game in particular, maybe Mario Kart or any first party game tbh.
Although I mentioned JUS it is really a though question, I enjoyed a lot the AA Trilogy in my DS and Metroid Prime Hunters online multiplayer as well as Mario Kart, I still think JUS is better because it is so niche and it hasn't yet a proper sequel unlike almost every other famous DS game.
JUS ;) and Time Hollow and Hotel Dusk (I know they have their fanbase, but I don't remember hidden gems for now).
Well, DS online gaming was peak online gaming for me, although it wasn't perfect I preferred it over, let's say Call of Duty on PS3, It really hit me when Nintendo closed their servers, gladly the community picked up right after with custom servers, and they keep working fine for most popular games.
I'm playing JUS online with the folks that want to play nowadays shamelessly sharing Discord group. but I'm also playing Chrono Trigger for first time ever.