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1) Nintendo DS. The system was so revolutionary and there were so many innovative and great games for it. If feel that in a few years (after the 3DS has been properly phased out) the DS library will grow in popularity again, as dual screen gaming is so unique. The DS had a lot more games that used its unique features than the 3DS. You can't scratch that itch anywhere else, only on DS (and 3DS, but that didn't make use of the dual screen, touch screen and microphone that much).
2) Gameboy (Color). The Gameboy was just amazing back in the day. Playing Mario, Zelda and later Pokémon on the go was great and the system had roughly a decade to build its impressive library.
3) Gameboy Advance. The GBA got many great titles and would be considered on of the all-time great consoles today if the DS hadn't been released so early. Great system.
4) 3DS. The weakest Nintendo handheld for me. It was based on a gimmick most people didn't want to use and I distinctly remember the original 3DS model being a step down in build quality (end even screen size) from the DSi. The 3DS has many great games but it didn't reach the heights of the other handhelds for me.
If the Switch was included it would probably tie with the DS for me right now. But it still has a lot of time on the market left.
