With a reasonable degree of confidence, Signal is the best secure messaging app. It's open source, audited, and e2e encrypted. Whatsapp started using Signal's encryption as well, but Signal doesn't share data with Facebook.
Also, a reminder that currently, Facebook can and does read Messenger's messages. It reads the text and also follows any links in the messages, and uses this info for advertising purposes.
My main concern with this move is that Facebook will start keeping and using the metadata (who you send messages to, and when), correlating it with other data (where you were when you sent those messages, whether or not you're FB friends with this person, what groups you're members of, your political leanings, etc), and using it for its own purposes. I am also concerned about Facebook outright breaking or backdooring the end to end encryption on Whatsapp and using the new backdoored encryption for all three services.