FaceTime seems to be the only service which delivers consistently good quality. Skype's video quality was abysmal the last time I used it and the Google app (Duo I think) was the worst ever - infact that one showed the recipient the video flipped vertically. I didn't bother to see if it would be resolved, but instead just uninstalled it. I don't use Whatsapp's audio calls, so I doubt I will use video.