I've experienced this very often: great GUI, good usability, and poor sound, and vice versa! It's very rare for a manufacturer to get both right. Most FM synthesizers sound too thin, and even the Halogen lacks bass.

The NI FM7 (32-bit) was a joy to use, and it also had many presets.

Another positive surprise is: Maxx Claster EJ VST - eJ is an FM synthesizer.

[Team BEAT | 09/23/2004 | 1.16 MB] 32-bit!

Features: – 6 operators, each with its own ADS envelope generator.
– ADSR amplitude EG.
– Resonant LP/HP filter with LFO and ADS envelope.
– 3 effects: chorus, echo and 6-band EQ.
– 450 presets.

The successor FM - 6 Operators slightly better:

Maxx Claster - Toxic II VSTi
Maxx Claster - Toxic III VSTi

 

FM8 is still good, though the UI could do with a revamp for HiDPI displays (but that's not gonna happen). That's my go to FM, and it imports DX patches as well so I don't need to fuck about with Dexed (which is great, do not get me wrong, but like DX7 it's not exactly fun to use). Every FM synth should have macro controls like FM8 and a spectrum display by law.

But Ableton's Operator is also fun to use and sounds great. I've tried Tracktion f.'em (ehh about the name) which is ridiculously powerful but you know, handling 4 ops is hard enough, 10+ operators is a task and a half to tame.

 

More user-friendly + FREE
Baconpaul - Six Sines
https://github.com/baconpaul/six-sines