My absolute favourite, and pretty much the only one I use for anything (on a slightly dated iPod touch):
http://megacurtis.strange.agency/ Granular madness. Has not been updated for newer iOS versions in the past two years or so. Have played entire sets using only this and an SP-404sx. Cannot recommend enough, and pray that they update for compatibility with newer versions of iOS. Well worth the $3 if you have an older iOS to run it on (9.3.5 or earlier I guess).