I voted WiiU as someone who hasn't tried DS emulation on the Switch. In general, it is a great system for nearly every platform that precedes it, with only the 3DS being fully incompatible. While I vastly prefer playing emulated DS games on my 3DS, the WiiU's ability to split DS games between the TV and gamepad for faithful dual-screen compatibility makes it my second favorite method and I doubt the Switch would trump the WiiU's novelty for me even if it still plays the games just fine. I remember hearing that the WiiU has a terrible N64 emulator but I rarely dabble with that system's library so I don't know how bad it really is or if some homebrew alternative fixes/replaces it.
On a technical level the Switch feels like the only system anyone should choose, especially if the 3DS can be emulated well and again the novelty of playing DS games on the TV/Gamepad combo doesn't intrigue you.
New 3DS is of course the best dedicated handheld for the job since it will pretty flawlessly handle any Nintendo handheld you throw at it. I don't know how playable N64 games are considering the limitations of the 3DS' analog stick and how notoriously difficult it can be to make N64 emulation feel natural on anything but an N64 controller, but the games can run well and that's awesome that it is an option for everything but Gamecube and onward Nintendo consoles.