Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
Korg Legacy Collection - Korg Legacy Collection is a suite of VST instruments that acts as an audio plugin software for integrating synthesizers and effects units into your digital audio workstation.
FL Studio Mobile - Create & save multi-track music projects on a smartphone
Dexed - Dexed is a multi-platform, multi-format plugin synth that is closely modeled on the Yamaha DX7.
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹