Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
Sampulator - Make (and record) beats on your keyboard
Logic Pro X - Music production.
cassini - Polyphonic Synthesizer for iPhone/iPod touch