The most popular and one of the world's first messengers, WatsApp. I personally use a version like Softgoza.
The main features available in WhatsApp:
- communicating via messages or calls (via the app);
- group chats;
- identification and display of the subscriber's location;
- integration of contacts from the phonebook;
- forwarding contact information to other users;
- personal customization of the interface design;
- customizable notifications;
- forwarding of correspondence history by e-mail;
- sending and receiving photos.

Security of communication via WhatsApp is protected by the end-to-end encryption protocol, which is enough for ordinary subscribers.
Of the disadvantages, it can be noted that one account is installed only on one mobile device (additionally - the desktop application), as it is linked to the phone number. According to user feedback, synchronization between the two versions does not always work correctly.