Unfortunately it's whatever the other person uses. So iMessage, WhatsApp, Instagram, line, and FB.

Signal Telegram Threema Briar Conversations for XMPP Element for Matrix Jami SimpleX

At the moment, as they come and go a bit as I'm testing out new ones.

I only message my spouse on a regular basis. Been using Signal for the last few years. Otherwise I just stick to iMessage for everything else. I don’t text anyone else that much to try and convince them to switch to Signal or another alternative.

Whatsapp and Google Messenger for RCS. I wish other apps could use RCS or I'd use third party.

Telegram almost exclusively, SMS for stubborn family, and Discord for stubborn communities that won't use something better.

I use the message app that came with my phone, lol. No one in my life seems to use anything else, either.

almost exclusively Snapchat, and a little bit of Telegram, then Teams and outlook for work.

I used fb messenger for anything communication in my family, friends, and my class. I kinda used discord, but non of my friends use it.

I sadly still use facebook as that's the only way I can keep up with family. Instagram because everyone who competes in my sport uses it, and lemmy.

https://movim.eu/ is a cool web-based XMPP client.