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.
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
FL Studio Mobile - Create & save multi-track music projects on a smartphone
Vital - Vital is a spectral warping wavetable synthesizer with drag'n'drop modulation workflow and animated preview of the synth's inner workings where needed. Comes with many modulation sources (including audio-rate), MPE support and FX chain.