I loved my DS the best of any non-PC handheld I have owned.
Final Fantasy 3 took up many many hours on car rides. Castlevania Portrait of Ruin is an all-time banger of a game, glad it finally got republished in a collection.
The first game I got on DS was Super Mario 64 DS, which, on top of having one of the finest minigame collections of any handheld game and being able to do single-card multi-player via download play, was a fine adaptation of one of the greatest platformer games ever made.
Brain Age and its offshoots spawned a whole cottage industry. Really, the DS was one of the first widely owned devices that had a decently reliable touch screen, so it got used for a lot of non-gaming stuff in addition to having such a huge library of games.
Pokemon Diamond, Pearl, and Platinum are the best of the classic top-down JRPG style Pokemon games IMO, so the DS also gets credit for having the peak of those games.
The original DS was also home to some of the best point and click adventure games of its era, like 999. This was before Telltale really took off with The Walking Dead, Batman, etc and the genre was mostly dead in the west at the time, so when some quirky Japanese point and click escape room/mystery games dropped it really was incredibly refreshing at the time. Those games still hold up IMO.
When the 3DS came out, I was a little disappointed by the StreetPass features. I live in a fairly rural area so I would only get to play Mii Adventure or whatever it was called when I would go into a city for a convention or something similar where you knew a large concentration of nerds was going to exist. I suppose it makes more sense in Japan with their higher population density. Regardless, the 3DS’ Gamecube-tier graphics, nicer buttons, better screen, and control stick all make it a superior machine to the DS in every iteration.
It’s really just a shame that Nintendo used the 3DS naming scheme. Like with the WiiU it led to consumer confusion where parents assumed it was just an upgrade on the original and not a whole new console generation. The naming implied it was just the next model after the DSi-XL and that all it added was 3D, rather than being Nintendo’s first properly online handheld and having a generational leap in raw power.
If I were going to buy a dual-screened handheld today, I’d probably go for the AYANEO Flip DS, which seems to be basically a next-gen Steam Deck but with the DS form factor. That said, it’s pretty pricey.