The Yamaha PSS 390 is the “toy keyboard” claimed to have the better and best sounding FM synth engine in the toy range.
It has 100 tones that can be widely modified with the tone controls, instead of button +/- it features sliders to edit the synth parameters.
The only lack is the midi control that however  can be added by retrofit.
It can be seen in action in the videos of the band “the Apparat Organ Quartet”.