I picked three options from the list:
GBA / SPThe first Nintendo handheld that I really felt attachment to. I'd had a Game Boy Colour before this but never really saw myself as a "Nintendo guy" and so in honest I never really embraced the machine in the way that I could have done and it always felt like little more than a minor distraction (with the notable exception of two of my favourite games on there: Link's Awakening and Wario Land
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As someone who was very much "Team Sega" during the 16-bit generation the thought of getting what was in many ways a portable SNES and having access to some of those amazing games that I had missed out on in my hand was a very exciting prospect! Playing the likes of Super Mario World, Link To The Past and things like the SNES era Final Fantasy games was a great way to catch up on some of the things that I had missed. Add to that some of the excellent GBA exclusive games like Metroid Fusion, the superb Mario Kart Super Circuit, the Sonic Advance series, Castlevania etc. and you have one truly excellent handheld machine!
SwitchFor me this is the best handheld machine there has ever been, simple as that. The breadth and depth of the software catalogue is insane, and the quality on display is staggering at times. I've played so many games on this machine that I could comfortably make a case for them being the best or my favourite in a series/genre: Breath of the Wild as best Zelda, Odyssey as best Mario, XC2 as best JRPG, Luigi's Mansion as best ghost hoovering simulator, Mario + Rabbids as best X-COM game that isn't X-COM etc. And that's just the exclusives, there are so many other fantastic multiplatform and indie games that I played on Switch and strongly associate with it - Sonic Mania, Streets of Rage 4, Axiom Verge, Hollow Knight, Stardew Valley, Dead Cells... the list could go on forever. And then there's the ports! I never owned a Wii U so this is just another vast collection of excellent games for me to experience for the first time! And I've forgotten about all the curious and unique experiences as well! Ring Fit, Labo, Game Builder - all doing something new and interesting that helps make the machine stand out and feel in some way special.
Another factor in how highly I regard the system is that lots of these games just feel better on Switch for me, the freedom to play handheld releasing them from the confines of the living room TV and letting me have so much more time with them. Playing late at night in bed harkening back to formative times as a kid staying up late to sit wide eyed in front of the CRT in my bedroom to play on the Mega Drive. In many ways I think the system in general has played into nostalgia very well, it's the easiest and most playful console I've experienced in a long time. It just works, it's always there ready to be turned on and instantly offer me a fun experience wherever I am - it really feels like being back in the glory days of cartridge console gaming where you were never more than a moment away from some fun.
It's also the system that has given me the most fun gaming with my kids - family multiplayer games of Mario Kart or Tennis Aces or the mini-games in Luigi's Mansion have been a highlight in our house. Playing through all of Odyssey as Cappy to help my son and daughter get through this massive adventure was just a brilliant time, introducing them to things like Zelda and Pokémon for the first time and watching them find their feet in these types of games... yeah, it's just been a great system for family gaming!
The Switch also allowed me to get involved with AYAMMM which has been absolute blast over the last two years and that is definitely the kind of experience that helps the system to reach even greater heights in my estimations!
Finally a somewhat leftfield choice, the underdog:
Game GearYes it ate batteries, yes it didn't have the best library of games, yes the Game Boy shat all over it, but I will never not love this beast of a handheld!
It was the first handheld machine I ever had, and the first machine that was actually mine and not shared with my brothers so it will always be special for that reason. I got this for Christmas when I was about 8 and I have such amazing memories of playing it over that period, I got it with Sonic 2 and Columns and basically spent all of the Christmas holidays playing them both and having an amazing time. Over the course of its life I never got a huge collection of games for it but I adored all of the ones I did have - as a big fan of the blue blur I had most of the Sonic games, they paled compared to their 16-bit equivalents but it was still amazing to have some kind of portable Sonic action, there was an excellent port of Lemmings for the system which I spent a hell of a lot of time playing, SEGA did some great conversions of many arcade and MD games to the system which I adored as well: Golden Axe, Streets of Rage, OutRun - all a hell of a lot of fun to play on the go. I also got the TV tuner adapter for it so would often use it as a portable TV which was another reason why I've developed a really strong attachment to the system - staying up later than I was allowed to so that I could watch films like Aliens and Predator for the first time on TV whilst hidden under the bedcovers!
So yeah, it's clearly never going to be a winner for most people but I just can't ever lose the love for the old GG, it was my first handheld love and nothing else can ever take that place!