I Like Telegram
Because Telegram is the first, the desktop messenger, and the easiest of all I tried. Telegram is the richest as it has everything I need to make online learning classroom. My case is unique as at that time (2014) I still had phone so I can register to it on my desktop and use it until today. Today I do not own phone nor number anymore. Important part of my reasoning was I disliked WhatsApp at that time which discriminated GNU users aside from being nonfree and could not be used on desktop. I say Telegram is ready. So here's why not the others from me.
Why not Signal?
I do not have phone while my main device is desktop. The reality is I cannot use Signal mainly on desktop while with Telegram I can. Signal is still poor in features and fanciness, just look at how Telegram presents everything to you (blog, news, features, channels, bots), so to me it is inferior. Even if I knew Signal at 2014, I would still choose Telegram anyway.
Why not Element?
Riot.im or now known as Element is not stable. Ironically, it becomes unusable because of its own security system (I admire Signal who can make this easy and usable). I and students cannot use it just like we use Telegram free from hassles. Compared to others, today this is the one closest to Telegram. However, even if Element was existed at 2014, I would still picked Telegram anyway. Element is not ready.
Why not XMPP?
Because XMPP is confusing for most people. For normal people point of view, we just want to communicate, not to choose different servers nor different clients nor involved in security talks. The only closer to Telegram from XMPP is Movim. However, it is still not as feature rich as Telegram. If only I knew XMPP or Movim at that time, I would still choose Telegram anyway. It is not ready.
Why not Jami?
I am a GNU Teacher so naturally I should prefer GNU applications but Jami is still not stable like Element. Too slow to send messages, too not reliable to share files, and not as feature rich as Telegram. Important feature of Telegram is web access and Jami does not have it. It is not ready.
Why not Session?
Like Signal, Session is also good free software. However, Session is too limited in features, would be too difficult to my students, and no web access as well.
Why not security, federation?
When you read this you will see I did not talk about security nor federation. So did most people. My purpose is focused on migrating people from nonfree to free software (mostly from WhatsApp to Telegram), I believe this is a noble job, and it is already a huge job heavy to bear. I do not want to add more focus. I do not focus on security or federation of course. It is a matter of strategy: once they felt free software, they will learn and develop to appreciate security and federation later – so let's not burden them and me with too much focuses. Let those carried by someone else.
I like Telegram.