IceGear have been making great synths for iOS since the very early days of the platform. One of their first apps was Argon, an excellent iPhone synth. From their they’ve continued to make excellent iOS apps for music makers, and right now they have a good selection of those on sale up until the 8th of October.
Here are the iOS synths on offer right now are:
Laplace is a physical-modeling synthesizer based on resonator synthesis that makes it easy to create bowed string, plucked string, blown pipe and metallic sounds. But it is designed not to reproduce real acoustic instruments but to create new synthesizer sounds in different new ways.
Laplace is on sale at $5.99 (usually $7.99)
Mersenne is quite unusual in my opinion and, I have to warn you, you can lose a lot of time messing around with this app. I have.
Mersenne is a melodic percussion synthesizer that combines two FM synthesis modules, a noise generator and a resonator. With simple operation, you can easily create a clear bell sound with complex attack sound.
Mersenne is on sale for $5.99 (usually $7.99)
Lorentz is not only a live-oriented analog modeling synthesizer, but also has a resonator that makes metallic and aggressive sounds. This makes it easy to create a new synthesizer sound with a resonator added to the sound like an analog synthesizer.
Lorentz is a universal app and is on sale for $5.99 (usually $7.99)
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.
Redshrike is also a universal app and is on sale for $5.99 (usually $7.99)