One of my favourite all time systems. So many great games, full BC with DS games, and with CFW you can open up an even wider world. My backlog is huge.
Something I always really appreciated was how versatile the 3DS is. It's got cameras (and can read QR codes), a microphone, a pedometer etc. A lot of 3DS games used these features in interesting ways, and you can feel the loss of these with some Switch sequels.
Plus you have fun features like StreetPass, Play Coins, themes, Badge Arcade, and tons of little tiny flourishes and Easter eggs. It's huge in personality, which is completely absent on the Switch.
I miss that Nintendo.