First of all these are all the apps which I personally used maybe your point of view is different but these are the apps that worked for me better than others which I didn't mentioned.

1- Google Duo

This is the latest app on the list. The headlining feature of this app is its simple user interface. You just need the phone number of the person and you are good to go. This app also has a feature called 'Knock Knock' where you can see the caller before you answer the call. It is a free app that is available for both android and ios.

2- Facebook Messenger

You might not think of messenger as a video calling app but it is. First when it was released it was just a messaging app but with time facebook added voice calling and video calling to the app. In my case this is the best app besides Google Duo for video calling. The video calling feature work on smartphones as well as computers. You just have to add the person on facebook and you are good to go. This app is free as well for both the operating systems.

3- WhatsApp

Just like Facebook messenger, whatsapp was not developed for video calling. It was developed to be the world's best messaging app and it is one of them. And after it introduced voice calling and video calling it became even better. You get everything from messaging to calling in one place. But in my experience it is not the best video call experience, messenger and duo are much better options.

4- Skype

This is one of the first video calling app on both computers and smartphones. This is not the best app for video calling but it was so famous and so many people know about it is a big strength. The video quality is a little bit choppy and there are constant stutters in audio which is also not ideal.