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.