Best overall: Telegram! Telegram is the best messaging app, but not the most secure. It is the most feature rich, has the most luxurious apps across all platforms, beautiful animations, and generally is wonderful. The app to copy. Suddenly gained a reputation for hosting white supremacist content, so now using Telegram is socially problematic. Voice calls are garbage quality.

Most secure: Signal. That's all. Phone number identity sucks. Sluggish. Feels terrible to use.

Second most secure: WhatsApp. Kinda bad. Sluggish. Poor features. Phone number identity sucks. Sluggish.

Discord: Slowly becoming the QQ of the west in every meaning of the word. Insecure. Only thing that makes it noteworthy is that everyone is using it. Totalitarian moderation system bans you for saying “im 12” as a joke. Voice calls are pretty good quality.

LINE: “Letter Sealing” security is ok. Functionally, security defeated by various backup systems like LYP backup. Has ads in Japan (even with LYP).

iMessage: Only secure if all of your friends have Advanced Data Protection enabled, which they don't. Screws up contact photos and contact data more often than not. Frequently sends messages from the wrong source. Routine cause of confusion. Sluggish.

WeChat: hands-down best automatic translation feature ever. Only app that makes communicating in foreign languages easy. Impossible to use with anyone outside of China who doesn't have Chinese friends, due to requiring existing users to invite new users. Risk of account bans for foreigners who invite problematic users.

Wire: Bad. Sluggish.

Viber: Bad. Sluggish.

Instagram: Insecure. Very fluid, though. Middling feature set.

Facebook Messenger: Middling feature set.

QQ: China only. Kinda overloaded app. Kind of sluggish. Overall nice social features. Very well-built.

Google chat products: Likely to shut down, get merged, or rebranded. Functionally dead post-Hangouts and Google Talk.

Slack: The best work chat app.

Microsoft Teams: If anyone forces you to use Microsoft Teams, I'm sorry.

KakaoTalk: So I haven't used KakaoTalk.