iOS Musician Apps (50) Instrument Sounds is Keyboards, Free

Spectrum is a collection of Audio Unit ports of popular eurorack synthesizer modules.

ToyTone is simple synthesizer. Select waveform, and move the slider. Tap the pad at bottom, you can play.

Made for complex and organic sounds, Factory sports a big modulation matrix with all kinds of features to create a wildlife of sounds. A vast array of DSP is ready to play: Analog style oscillators and filters, but also different flavors of wavetable synthesis, Wavetable driven Waveguide resonators and a 3-FX-Chain in a dynamic signal flow.

Aparillo is an advanced 16 voice FM synth tailored for complex sound structures , majestic motions, moving and evolving soundscapes and atmospheres that are perfect for film, TV or games.

A Simple Ad-Free Piano-like Musical Keyboard. Easy to Play, Good Response and Small Application Size. Just Play the Instrument. The app doesn't use sampled real sounds but use FM synthesis. It sounds like piano though it's false.

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

iPhilicorda is an emulation of a vintage organ from the sixties, the Philicorda, created by the Dutch company Philips.

Syntorial is a synthesizer training app, that will teach you how to program synth patches by ear.

Accelerometer Synthesizer and Bluetooth MIDI Controller.

LCW-2 mono is a little complicated synthesizer based on ToyTone and implements several experimental functions.