Recs.
Updated
SpecsUpdate
Pros
Pro Cloud data sync
Telegram synchronizes all data in the cloud for easy access to all your files and conversations on any device. Only for unencrypted chats. Encrypted chats are single device to single device because there is no cloud storage, and rightly so.
Pro Secure
Pro Banned in anti-democratic countries as China, Iran, and Russia
This is a good proxy to show that the encryption actually works!
It shows that these governments cannot break the encryption and that Telegram does not collaborate with such regimes.
Pro Compelling story
The founder Pavel Durov was exiled from Russia after refusing to cooperate with the Kreml to stamp the opposition and oppress the people. He nearly lost everything he worked on and started Telegram in exile. He has been a big proponent of free speech and unquestioned privacy since then.
https://www.bloomberg.com/news/articles/2017-12-12/cryptic-russian-crusader-says-his-5-billion-app-can-t-be-bought
Pro Save time with inline bots and link previews
Unfortunately, on the desktop videos don't play, but images are viewable without visiting popular websites like imgur and Instagram.
You can play YouTube videos and others within the app, so you don't need to embed code or anything.
With inline bots, such as @gif, @pic/bing, @vid/YouTube, etc, you can look things up without leaving the client or chat that you're in. Simply type the name and some keywords and wait for the results to load.
Pro Two-step verification
You can set up an additional password to your account, which can be reset by your email. You don't have to use an email if you do not want to. If you lose access to your email or forget your password, you will be able to reset it via text message if your account is over seven days old and will receive it within a week (in case you recover anything before then).
Pro Supergroups of up to 100,000 people
In Supergroups, anyone along with admins can delete their messages from all sides. A message can be pinned to the top. All of the user's messages can be deleted by an administrator, and they can be banned and reported as well. Supergroups can have a public link (outside of Asia, due to high spam volumes), which is also a username.
Pro Cloud chat keys are kept in different jurisdiction
Even though encryption used for cloud-based storage (e.g. Gmail, Facebook, iCloud) may not be as secure as end-to-end encryption or zero-knowledge storage, Telegram does the most that it can by holding keys to chats in different jurisdictions, requiring intergovernmental cooperation to obtain the keys. This is aside from whatever other security measures have been taken, of course.
Cons
Con Their own encrypting engine
It's questionable how secure their system is. They've developed their own encrypting - and apparently none of the people behind the service are experts in that field.
Con "Secret Chat" feature is not available for desktop client
Secret chats with end to end encryption are only available on mobile apps.
Con Doesn't have video calls
Very powerful and secure app. Nowadays it's not only a messenger.