My suggestion, btw: Delta Chat.
How does Viber stand according to the source? (or did you search the messengers individually)
I remember it being pretty popular somewhere like India or sth
Thatās very much true about me 
if you look at the market domination of apple in the US then it is definitely iMessage there. search for blue bubble vs green bubble on YT
thatās why the whole RCS as secure cross walled garden chat is such a big topic there.
Supposedly, FB Messenger is at 55%, iMessage at 29.
India probably helps the Whatsapp totals: it is the default message app here.
This is why the very few people that I know who insist on Signal, rather than Whatsapp, do so for stuff they really donāt want to have spied on.
Of course, IIRC, Whatsapp and Signal have the same authors, so the security must be similar.
We have to trust somebody. And if I was doing anything more sensitive than day-to-day chat and sending pics of musicians to them, then Iād have to make that decision and, at least between these two, Iād go for Signal ā not whatever Meta might have done to the original privacy promises made for Whatapp.
The Problem with signal is that there is also one central server which is located in the same country as WhatsApp and fb Messenger.
I am sure the signal authors would punch you for this statement.
WhatsApp uses an encryption scheme based on what Signal does. thatās correct. but all the garbage that Meta does around the e2e encryption, you dont want blame any developers beside facebook for that 
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That central server stores a very few data points. the list of them is public.
The real valid critical point for signal is: they use some binary only blobs for certain functionality and it would be nice if that could be replaced with opensource code.
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They do not. WhatsApp founder came to signal well after it was already going.