I think you may get a number of answers from the fitness professionals here–there are many different apps out there and each trainer has a their own individual preference.
If you have a membership to IDEA, then one option is to use Workout Builder to create your workouts and send them to clients. It allows you to make notes on each exercise, and then again when you actually e-mail the workout to your client. I use it a lot and clients enjoy having the videos. Plus, it’s very easy to use.
Others will have more ideas, but I use Workout Builder and have been happy with it.