So far, these have been my results:
Skype: Very, very good when the internet connection is perfect. If it's less than perfect, look for alternatives.
Zoom: Better than Skype in terms of stability, but has worse sound quality than Skype. It has the BEST screen sharing options. However, it can freeze with some connections. Also, the chat options can be confusing. Make sure to always use the contact chat instead of the meeting chat if you want it to still be there for the next lesson.
Google Hangouts: In general, it's the most stable, and dead simple to use. However, video can freeze sometimes, even with good connections.
WeChat: Better for Chinese students. No screen sharing option.
FaceTime: Very good quality. Good for Apple users because you can chat on one device and send images with another. No screen sharing option.
What have your experiences been?
Normally I use Skype for classes but also have used Zoom (also for group classes) and occasionally Google Hangouts. I prefer Zoom but not all teachers offer it. I find screensharing quite important and for corrections and notes, I like Google docs. Some languages (e.g. are difficult to read in Skype).
@John: The free version allows calls of unlimited length between TWO people.
If you want a group call, then the 40 min limit applies.
