Telegram is not end-to-end encrypted by default, allowing the
Telegram server to see all of your data unless you use a "Secret Chat". Telegram uses custom, unaudited encryption. The first
version of MTProto had severe security issues but these were fixed with MTProto 2.0. Telegram uses odd cryptographic
primitives such as AES-IGE for "performance", although they use it in a way that they aren't affected
by its security issues. Telegram has also been criticised by well known cryptographers such as Moxie Marlinspike
and Matthew Green.
Telegram has held crypto cracking contests but these were rigged.
Although the clients are open source, the server is not.
The creators of Telegram have spread baseless misinformation about competing apps before.
Telegram, along with most other messengers, leak metadata about your messages, even if the message itself was end-to-end encrypted.
Silence is dead and the only commits for a long time have been translations, not code updates.
Wire stores all metadata
unencrypted and is owned by a US company, a country known for privacy abuses.
Unlike Telegram however, Wire has been audited and its server code is fully open source, allowing it to be self-hosted.
Use Signal. It uses solid encryption, has a great track record, is recommended by many experts in the field and a lot more. Most other messengers are subpar.