IMO FM8 is the best by a mile.

To me, you give up a little in basically sound quality(and that is probably debatable or depends on your taste) for a massively improved programming interface.

You can't go back to a hardware interface after that unless you just want to stick with presets.

Strangely enough, I'm not a synth guy, so I don't have favorites to recommend :-)

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.

The Buchla 258 oscillator has amazing fm that sounds alive.