I'm using Telegram since 1.5 years now. I used WhatsApp and iMessage (non US country), tried to use Signal and yet, featurewise I did not find any of them better. Telegram has MtProto Security framework, which is not the best secure answer, but it still is for people who want to be safe. This is on secret chats and the reason is simple. They are not stored in servers, so you can only access the chat from your and your friend's device who you are texting with. If I would trust either WhatsApp or Telegram, I would easily choose Telegram. Telegram's founder is Russian, yet none of the servers are in Russia and he is anti-Putin in his speeches and so he moved the entire team into Berlin and Dubai locations where it won't be suppressed by those governments. In my 10 years of computing experiences, you can't be 100% secure when you're messaging. Even if you try to protect yourself, your friends will not and thus it won't worth it. Based on your choice, either your ISP, your cellular or your vpn company already has your data.
Feature wise, it has so many features and so "better" features than other apps, so my main app would be Telegram. I still have to use WhatsApp for Business and for family members, so I have both of them by must.