SpaceLab (App Store link) is a monophonic synthesizer for iOS.
It has four ways to play: keyboard, graph, guitar, and wind controller. Underneath those interfaces is a fully programmable subtractive synth with oscillators, filters, envelopes, effects, FM, presets, an arpeggiator and full MIDI control.
It’s $.99 in the App Store. Demo below.
Here’s the SpaceLab take on the Theme from Midnight Express: