3DS.
Though I didn’t have many - GBA, Wii, 3DS, and now Steam Deck. The Deck is much better in a vacuum, but the 3DS just offers an experience you couldn’t get via emulation or ports. Even if it’s just the dual screen, it looks better as seperate physical screens than what emulators do on single display devices.
With the Deck, I just got it as an alternative to buying a new gaming laptop. Years from now, I’ll be able to replace my SD with another Steam, or even non-Steam handheld PC, but once I lose my 3DS and functioning second-hand ones run out too, that experience is gone, and even 3rd party emulation machines will probably lack features.