Mine is Yamaha DX7-II. Such amazing emulations 100% synthed (no samples). I love the digital pianos, slap bass, harmonica, brass, mallets... Strings are not that good, although they are way better than on the original DX7. Even the acoustic piano patch is interesting while its really far from what it tries to emulate.
The DX7 was quite an engineering achievement. Waveforms in the log domain so an add is a multiply, lots of tricks to get the FM algorithms to fit in those early crude ICs.
Can't remember where I read it, but IIRC someone was flown in to make the initial presets and only give some ridiculously short period of time to make them in, like a day or a week or something. Given how non-intuitively the FM parameters map to output timbre, that must have been a huge headache.
I owned a used FS1r for a couple of years and sold it on eBay for more than I bought it for. It had a typically cryptic Yamaha UI which made the front panel controls almost pointless. Format implementation seemed like an afterthought. Seems like they were just going through the motions in the design phase, which is too bad, it could have been a real monster.