Not a tracker, but Drambo is a very hyped app for a reason. People say it's somewhat an easy-to-use octatrack married with a flexible modular synthrack. Been never attracted to either of the two. But I recently downloaded it for a train ride. And man it rocks!!
Great combination of sequencing tr-style, note sequencing and piano roll. Very good sounding modular that abandons cables for a good reasons. But also with sampling you can go wild. Didn't even feel 9 hours passing