What on earth is Threema? i know what signal is.
Myself, I just use good old SMS. None of this RCS rubbish either, if I had an Iphone, I would default it to SMS, saying that, I only text a couple of people that have Iphones.
Threema is an end-to-end encrypted messenger that takes security even more seriously than Signal. When you add friends remotely, they are shown with a single "LED" in red, you can then do various levels of verification (to get 3 green "LEDs", you need to come together and scan QR codes from each other's devices) from secure (red light) through known (2 orange LEDs) to verified (3 green LEDs).
In Europe hardly anyone uses SMS for regular communications, despite over 20 years of flat-rate SMS. Most use WhatsApp, Telegram or Signal. iMessage falls way behind in most circles as well - mainly due to the low penetration of the iPhone when the messengers appeared. When WhatsApp and Signal became popular, here in Germany, the iPhone had less than 20% market share, so iMessage users were in a minority and Android just had SMS, so people gravitated to WhatsApp or Signal.
From a security standpoint, the list probably goes Threema, Signal, WhatsApp, Telegram, iMessage, RCS and SMS.
iMessage falls so far back in the list, even though it is probably equivalent to Signal because it doesn't offer encrypted communications with Android devices, or only if they support RCS with encryption and many carriers don't, the encryption on RCS isn't standard, it is an extension that Google tried to push through but not all carriers use it.
Telegram uses its own encryption, so not verified, end-to-end encryption isn't turned on by default and only available on 1-1 conversations or small groups. Larger groups are encrypted to the server only.
WhatsApp biggest problem is that it uploads all of the contact information to Meta's servers in the USA - name, address, email, telephone numbers etc. The actual communication is end to end encrypted, using the Signal protocol. That makes it non-compliant with GDPR, as you cannot upload the contact information without first getting permission from all of your contacts and it can't be uploaded to non-EU servers... So it is quasi illegal, but so popular that it would be almost impossible to go after each user individually and prosecute them. There is a WhatsApp for Business that doesn't upload the data, to try and get around this problem.
Threema and WhatsApp are pretty much equal, both can verify the other side of the conversation. They also don't upload all the contact information, they just upload a hash of the telephone number of the contacts and if someone joins with the same hash, both parties are informed that they are on Signal. I'm not sure how that works now that Signal allows non-telephone number accounts, I haven't tried, but you probably have to contact people out-of-band and add them manually...
