Yamaha FM Essential

[Cinebient]

„FM Essential“ von Yamaha Corporation
https://appsto.re/de/ez55bb.i
Free!
It's also an iOS AU app.
Nice sounds. Sadly you just have access to all the goodies if you buy some hardware from Yamaha.

[Numanoid]

FM Essential is an iOS synthesizer app with legendary Yamaha FM synthesizer technology from the DX100, TX81Z and V50.

Connect FM Essential to an MX BK/BU synthesizer and unlock a full set of 271 Voices

Some impressive demo vids

http://usa.yamaha.com/products/apps/fm_essential/