The safest bet is to go for Signal. It's highly audited and proven true by their warrant canary. it has post-quantum encryption and a baseline encryption standard that is, quite genuinely, THE gold standard of encrypted messaging. It encrypts almost all metadata and attempts to mitigate privacy on areas where it can't encrypt/remove metadata, such as their sealed sender feature (which makes it so any observers of the network can't know who a message was sent from, only where it's sent to).

On top of that, it also has a great feature set. Most of the better messaging apps like Session or SimpleX might be good for anonymity, but they both lack many of the features people like in modern messaging apps. Signal has a lot of customization options and even has a Snapchat-like stories feature for people that care about that.

So yeah, at the end of the day, Signal would be my recommendation for people fleeing Telegram. It's generally best as a one-to-one messenger for people in your contacts list but it can also be used for group chats. If you absolutely need large-scale communities like Telegram groups, you might also check out Matrix through the Element client, but it's not nearly as feature-rich and only the messages themselves are encrypted; all metadata remains unencrypted. Signal is the best balance of security and features.