iOS Musician Apps (117) Supported Technologies is Inter-App Audio (IAA), Instrument Type is Synth

Moebius Lab is a multimorphic sound processor which contains a synth, a varispeed stereo sample player, effects and LFOs that can modulate each parameter.

Flynth is a synthesizer by Fingerlab, designed for musicality and easy use.

AudioKit Synth One is a completely free & open-source professional iOS Synthesizer. 

Pure Acid is a bassline synthesizer and drum machine in one app.

SunVox is a small and fast cross-platform modular synthesizer with pattern based sequencer (tracker).

Tardigrain is a granular synthesizer. Sounds are created by taking smaller fragments of a sample and playing them back in a non linear fashion, with settings for grain size, direction, step size and more.

A matrix-controlled arpeggiator designed to play with your thumbs.

Pixound provides a deep musical experience that lets you instantly create music using any picture.

PixiTracker 1BIT is a simple and fun tool to quickly create musical sketches, chiptunes and experiments with TRUE 1-BIT SOUND (like on old microcomputers).
No musical knowledge required.

Multilevel Interactive Sound Synthesizer. Up to 240 Frequency Modulation Oscillators or simple Wavetable Additive.

mood is a personal interpretation of the legendary Analog Synthesizer by Eugenio Giordani

Xynthesizr is a streamlined yet capable 32-step matrix sequencer/monotimbral synthesizer with generative features and MIDI.

GarageBand is a DAW by Apple.

Just a Toy Synthesizer. Included: 2 Oscillators, LPF, Delay effector, Ribbon controller.

KQ MiniSynth is a polyphonic modular synthesizer.

Thanks to it's true VA synthesis engine, the use of analog-modelled non-linear filters and extensive modulation options, Flowtones sounds warmer and more analog than digital romplers and wavetable-based synths.

Lagrange synthesizer has two oscillators, a noise generator and two filters that can be selected from various algorithmic models.

Mersenne is a melodic percussion synthesizer that combines two FM synthesis modules, a noise generator and a resonator.

Redshrike is a polyphonic synthesizer that combines a resonator designed to make more complicated sound creation and subtraction synthesis with an FM oscillator that creates violent distortion.