Whatsapp. I know signal, simplex, matrix, (a billion other things), etc exist that are much better, but where I live, no one uses them, for context, basically everyone, like if you have a phone, you use whatsapp, some government things even happen through whatsapp bots, when people say the word message here, they mean whatsapp. There are about 20-30% (among younger folks) who use telegram, but that is mostly for easier piracy, and larger file sharing (before whatsapp allowed 2 GiB, now they do it habitually). My mom has about a 1000 contacts, and less than 10 of them actually use signal (there are many more who signed up(there was another thing, basically when elon said "just use signal"))

At this point it is not worth getting everyone to switch, the best i have done, is just reducing the number people i communicate with (on whatsapp), and try to just meet in person