Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
cassini - Polyphonic Synthesizer for iPhone/iPod touch
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.
zMors Modular - Full featured modular synthesizer for iOS
SynthMaster One - Powerful, universal wavetable synth with an intuitive workflow.