SunVox is surprisingly versatile and good sounding, it's easily the best 'synth' on Android. It actually has several synth types, virtual analog, FM, samples, drum machine and more. Half a dozen effects. Sequencer. Modular signal routing. Excellent interface that's usable even on small-screen devices. To be honest I've only scratched the surface of what you can do with it but there's a lot of possibilities.
edit: Check the video on this page: https://play.google.com/store/apps/d...o.sunvox&hl=en
Electrum Drum is a fun drum machine to play with, does sampling, has some basic effects, includes access to a bunch of free sample packs.
TouchDAW is the best DAW controller I've tried. Can be a pain to set up but works surprisingly well once it's going.