Mendel Synth is outdated and blurred a bit on modern devices (never even got a retina upgrade for the iPhone 4), but it makes some wicked sounds. You mutate various sounds on assorted bubbles nearby, and make some crazy stuff you can then chop up, etc. You can email the raw sounds to your computer, too, if you like. Tbh, I don’t know if I’d still buy it today since there’s no telling when an iOS update will finally kill it.
Bitwiz is great fun for lofi bleeps and drones.
Purple Goo is like having Paul’s Extreme Stretch with a sort of resynthesizer on top, and it’s free.
Xynthesizr just became free.