Recs.
Updated
SpecsUpdate
Pros
Pro Truly cross-platform and open API
Not only does Telegram have an Android, iOS and WP app, but it also has a client for desktops on Windows, OSX, and Linux. Basically any device one owns, Telegram has a client that runs on it. Has a web app that changes its appearance based on the device it's running on and if its browser is full screen, etc.
Pro Secure
Pro 2-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 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.
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 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.
Cons
Recommendations
Comments
Flagged Pros + Cons
Pro Cross-platform
Works on Windows, Mac, Linux, Android iOS, Windows Phone, and Firefox OS even. Has a web app that changes it's appearance based on the device it's running on and if it's browser is full screen, etc.
Pro Free and convenient large file sharing
Copy/paste videos, gifs, images, geolocation, and documents, drag over files for practically any time up to 1.5GB in size. Images can be compressed, too.
Con Not fully open source
Telegram has an open API and protocol free for everyone, but the backend software is not open source. Therefore 100% privacy can not be evaluated.
Out of Date Pros + Cons
Con Not open source
Telegram has an open API and protocol free for everyone, but the backend software is not open source. Therefore 100% privacy can not be evaluated.