Whatsapp is huge. There is no argument against that. Everyone who has an Android phone is using whatsapp.
And this post is not about why you should stop using whatsapp. This post is about why I love Telegram Messenger.
Telegram Messenger is open for anyone to crack, or hack, clone, and improve. This is the biggest reason why it is the best among all messenger apps.
Telegram supports multiple devices simultaneously for the same account. This is possible because all your messages are stored on the cloud.
This gives you two advantages - you never need to back up your messages, and you can move between your phone, laptop, tablet, whatever and continue your conversations where you left off.
Telegram and whatsapp are like a metallic lunch box and paper wrap respectively, when it comes to security. Telegram even allows you to encrypt conversations such that only the recipient can read it.
Telegram allows you to send files. You can share pdf, mp3, doc, ppt, all those files you want to quickly send to a friend without having to resort to email or without using a pen drive.
Groups on telegram can be up to 200 members. Anyone can add new members.
Free as in free water. The people behind telegram is the people behind vk.com, the world's second largest social network. They have enough money to keep telegram running free for practically long enough.
Fast, though it is always arguable.
And the best for the last,
Availability on multiple platforms
Telegram has an official Android version and iPhone version only. But due to its open nature it has countless windows phone versions, a web version, a windows desktop version, and even a linux cli version mentioned on its website. That is not to mention the fact that you could develop your own client using the open source protocol.
In fact, I even built a bot based on telegram.
Give telegram at whirl, checkout telegram.org