How serendipitous to see this thread pop up not even a full day after I modded one of my 3DS's! I ended up having a pretty botched repair job from Nintendo when I sent a few of my consoles in, the tl;dr being that I had my cool n3DS consoles replaced with n2DS's, and all of the software I bought over the years wiped off of my console. Retrieving that lost software wasn't impossible per se but contingent on relying on Nintendo's now very gutted and unreliable 3DS customer support... Needless to say that was a kick in the pants for me to finally get around to modding one of my consoles and get all of my digital downloads back along with plenty of extras. Yay piracy!!
I dunno if the 3DS is my favorite console but it's definitely my favorite. It's one of the few consoles I had as a child which followed me into adulthood. Although I eventually recovered all of my childhood game consoles once I became an adult and video game collecting became a huge hobby of mine, 3DS was something I always had at least one working console of for over a decade by now, so I've always thought of the console as an ol' reliable and something I can always come back to whenever I need it, even if it's not exactly something I play the most often LOL. you get what I mean? That is my emotional support video game console :P
1. Anyhow, my first ever 3DS game was Pokemon X! That's right, I was one of *those* kids who got into the 3DS on the Christmas of 2013 with the new Pokemon game. I remember Pokemon X being the first video game I followed news of pre-release, watching the game like a hawk thru videos TheJwittz and Tama Hero (then TamashiiHiroka) made throughout the year. Honestly no idea how I was doing that, the 3DS was also my first ever device with a web browser and YouTube, but I remained on the pulse nevertheless. I remember having a lot of fun with this game, though I distinctly remember feeling incredibly deflated when I beat the story mode campaign not even three full days later. I ended up growing out of Pokemon by 2017, around the Ultra Sun/Moon era, and besides that I wouldn't consider X/Y to be the best of the Pokemon games I played, but I still have a ton of nostalgia for gen 6 anyway. One day I'll move to Lumiose City, I'm sure
2.Kinda hard to answer this actually hahaha. As long as I owned the 3DS and as much as I played it, I actually didn't play a ton of 3DS games. I was feasting a lot on those five dollar virtual console games and a lot of my favorite series, like Mega Man, missed out on proper 3DS titles but had plenty of stuff on the VC to dine on. Let's see... Smash for 3DS and Hyrule Warriors Legends were games I got hundreds of hours worth of crazy good fun but for the most part better versions of those games exist on Switch so calling them my current favorite 3DS game doesn't feel exactly right

Kirby Planet Robobot and Fire Emblem Fates: Conquest are proper 3DS games which I fell in love with and even played to completion, which felt like huge accomplishments for me at the time! And of course there's also Animal Crossing New Leaf to consider as well...
3. Project X Zone!!! Not a ton of people ever talk about this one, it's a strategy-rpg (so in the same genre as Fire Emblem, but it plays very differently!) and the whole thing with this one is that it's a crossover between a bunch of Capcom, Namco, and Sega characters. Each unit on your team is a pair of characters, usually from the same game (but not always; Frank West and Hsien-Ko are an iconic duo here), and when they engage combat you use button commands to perform very flashy, cinematic attacks with the intent on stringing them together in such a way that you keep the enemy airborne to maximize damage. It's so hard to explain, try to look up a gameplay video of it to see it in action!
4. I change themes a lot, but the Hyrule Warriors Legends theme is pretty cooool...
5. Weirdest game I've had the misfortune of experiencing on 3DS is Mario Maker. I'm sure at a glance Mario Maker doesn't sound so freakish and strange but on 3DS you can't share levels you make online, and there's no way to search for player-made levels to play, you instead have to wholly rely on whatever the "recommended" bar decided to feed you. I sold my copy of Pokemon Black to buy this damned game in 2017, if only I could go back in time and stop that from happening LOL