Probably Signal would have been a safe bet. Telegram doesn't do encryption by default (on group messages? Been a year or two since I've used it). And Facebook complies with law enforcement agencies, and I don't think it's unreasonable for them to have a feature flags to selectively and transparently disable encryption for some participants if need be.

Facebook certainly likes to at least have sense to know what you are conversating about. Sometime in 2016 we and my buddy abroad got our accounts frozen "due to security reasons" at exact same time; what we were doing is having fun with FB Messenger and sending each other PGP-encrypted messages. This least about 2 months and my buddy is Egyptian, so I am pretty sure at some point FB said "we don't know what they chat about and enough is enough". I got my account recovered after multiple layers of verification including video-call to hold up my ID done by third-party ... my friend never gotten his reinstated.