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
cassini - Polyphonic Synthesizer for iPhone/iPod touch
Logic Pro X - Music production.
zMors Modular - Full featured modular synthesizer for iOS