Im relatively new to iOS synths, but Magellan Jr and Sunrizer are the two that I would consider "must owns" (and both are under $5). They have similar interfaces and a ton of great presets and functionality. Magellan covers all the bases from leads to bass to percussive synths and effects. I also really like the SPAWN functionality, which is like a random synth generator - great source of inspiration. The various presets sound warm and powerful, and the act plays great with DAWs and other apps thanks to AudioBus and IAA support. Really just scratching the surface of what this app can do, but highly recommend it!
HouseOfHomuth about Magellan Jr