I use All-in-Yoga, but honestly I mostly appreciate it for the ability to create your own sequence and have it lead you through it, rather than the clarity of its directions. I haven't really had any problems following it, though, and I'm a base beginner.

IIRC the Hatha Yoga app is pretty good, too. I think its directions are slightly better? To be honest, I rely more on pictures and the fact that creating your own sequence gives you the opportunity to look up how to do each pose with both pictures and words and occasionally illustrated pictures of what muscle groups work in this pose before actually doing it.