Not the author, but I'll answer as someone who has owned many of Yamaha's classic FM synths. Without a doubt I think that the original DX7 Mk1 is the most interesting. The latter 6-operator FM synths such as the DX7 Mk2, or the SY77/99 were definitely more advanced, and had a 'cleaner' sound, but they don't capture the same classic gritty quality of the original.
