I've only recently gotten back to using Wire and am really loving it so far. Looks like they're doing everything correctly. Well implemented end-to-end encryption, cross-platform support and everything is open source. On top of that the tech is cool with a backend written in Haskell and Axolotl reimplemented in Rust. And they're planning federation support, which is just amazingly cool!

I saw stuff regarding the integration API a few days ago. It's still very beta as far as I can tell, but projects like this make it look really promising!