Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
ReSlice - Slice your audio with an advanced, rhythmic arpeggiator and FX.
Sampulator - Make (and record) beats on your keyboard
Ramsophone - A generative art/music machine. (Be sure to refresh!)
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹