GBA has the benefit of handling all games from the OG to the GBA games without being too bulky like the DS line. Plus GbA has some great re-editions of SNES games as well as Pokémon FireRed which is a nice revisit of the OG Pokemon (in my eyes). The backlight was awesome! The lack of headphones, not so much but it prepared me for the future (looking at you, Apple). The original GBA shape was much nicer though. I wish they had made a landscape console with backlight.
Luckily the GB(A) consoles are the easiest to mod nowadays, to add backlight, rechargeable battery, better audio... IMHO this is still the best way to enjoy GB/GBA games, instead of emulators which can be buggy/force me to stay on the computer (or worse, on my phone)
Got to be a toss up between the OG GBA, which could play original GameBoy games as well as GBA games, or perhaps the OG DS which could play 2DS and GBA games (although I don't think original GameBoy worked).
Or the Switch, which lets you play a full sized console Zelda game on the toilet.
I am fixing up a pile of dead DSi XL’s and let me tell you, those things are a joy to play on for my aging eyes and massive hands. They’re hell to get parts for (the battery cover, and the OEM type stylus with a softer nib rather than the cheap solid one piece plastic replacements, are basically unobtainium). But my preferred format for anything NDS/DSiware.