Dexed

The best free FM synthesizer right now is Dexed. It accurately emulates the most famous FM keyboard ever made, the Yamaha DX7. The emulation is so precise that the plugin can load original DX7 patches. If you have a DX7 patch editor and a SysEx manager handy, you can import sounds directly. There’s also a huge community of DX7 preset creators online, so you’ll never run out of sounds to try. The feature set closely follows the Yamaha DX7, with the added perk of all 144 synth engine parameters being available for DAW automation.

The user interface can take some time to get used to, especially if you aren’t familiar with FM synthesis. FM programming is a different workflow from subtractive synthesis, so expect a learning curve. That said, the classic FM sounds (electric pianos, bells, metallic basses, glassy pads) are hard to replicate with other synthesis methods. If those sounds are in your production style, Dexed is worth the time investment.

If you find FM synthesis too complicated but want to give it a try, check out Digits 2, a free phase distortion synthesizer that emulates the Casio CZ-101.