Well... I've had quite a few FM synths over the years... ORLA DSE-24, DX7II, TX7, TX81Z, FS1R, TG77, FB-01, TG33, TX802... maybe even more, I don't remember.
If you want TRUE FM synthesis, you'd go for one of the older ones... they sound considerably warmer than the newer ones, and even the newer ones sound better than any software I've heard... much more punchy, and with much less aliasing.
The reason is that they all use custom FM chips... operating in megahertz rates, instead off all the newer software bandlimited ones running at 48Khz...
Also... many of the newer ones that offer more synthesis features like P12, even though they can sound FM'ish, they are usualy harder to program, than a real FM synth architecture... just try and create feedback-loops with the P12... it's not that easy, and you have to think in another way than when programing a real FM synth.
Also.. as I mentioned earlier, programing IS hard, and the outcome is rarely what you want it to be, so having an arsenal of other peoples presets is key (for me at least), since most other has very few... so if you go for these newer synths, you will have to start programing if you want a lot of options... that's why I myself, would go for a simple TX802 any day... it has 16bit DAC, it sounds warm, it's DX7 compatible and use custom FM chips.