I've had positive experiences with both Microsoft Teams (Skype) video and with Google Meet in the last 6 months. In terms of "stands alone easily," Google Meet has a pretty great experience for the casual user, although it does have some confusingly implemented UX (for screen-sharing, admitting-users, etc). The UX is easy for tech-oriented folks but your grandma might need some training.
The one I was kinda surprised wasn't any good, even for audio-only, was Discord. Not that it fits your criteria anyway.
n of 1 and all that, I don't have any supporting data.