Somewhere between WhatsApp and Signal. It has FOSS clients, hands over user data only under extraordinary circumstances (terrorism and child abuse, afaik), and runs on pretty much any hardware. The last two points make it very popular in eastern Europe and most of Asia. The main problem with Telegram is that normal chats are not end to end encrypted, and instead use a weaker encryption algorithm. Secure chats are e2e encrypted, but are not on by default.
Overall, it is used by opposition parties in countries like Russia, Belarus and Iran for day to day stuff, so it is fairly secure. Of course, if you are a reporter or activist who has a lot of enemies, you could get something even more secure.