Whatsapp is the leading IM service.
I use this app for its fast performance even in slow networks.
The interface is great and simple to interact with.
I use it over other apps because of its speed, its simplicity, and since I have most of my friends on Whatsapp compared to other IM services.
It still misses some features which I would have loved to see like stickers. The emoji have improved now with better organisation and more emojis added.
The end to end encryption feature is a great one. I am very happy to see my privacy maintained even after Facebook bought the company.
Overall I would say it is my default IM service I use everyday of my life.
IM-Instant Messaging