Because private and group messaging play such an important part in our daily lives and because we often share confidential information in this format, it’s important to ensure the apps we use for this are secure - meaning they cannot be read by 3rd parties or are not open to what is know as man-in-the-middle attacks.
Security is achieved with something know as end-to-end encryption (see pic)
Based on this benchmark here is a list of the current most secure Messaging apps:
1. Signal 2. Threema 3. Viber 4. Line 5. Telegram  6. WhatsApp  7. Wire 8. iMessage 
Not on the List:
1. Twitter DM 2. Facebook Messenger  3. Instagram DM 4. LinkedIn DM 5. Skype 
 There have been some know issues with Telegram which have been fixed. It’s been reported that the team lacks deep experience with cryptography also note that end-to-end encryption is not on by default and you need to turn this on.
 Facebook owns WhatsApp and there were recent high profile departures from the team. Because of Facebook’s appalling security & privacy record it would be important to keep an eye on how the app changes going forward.
 If you backup your iMessages to iCloud then there is exposure if your iCloud account is ever hacked.
 Facebook Messenger does have a “Secret Message” option which does offer end-to-end encryption similar to WhatsApp
 Skype has a private message option but it‘s not on by default
NOTE: It’s important to understand that no messaging app is 100% secure and is also only as secure as the receiving party as messages can be screenshot or photographed.
NOTE 2: It is extremely important to keep any apps you use updated. Developers regularly update apps to make them more secure and patch any issues. Make it your goal to always have the latest operating system & apps.
Are you aware of security when messaging? Which app do you use? Let me know in the comments!