For most of the people I interact with, and most of the usecases: Viber

i consider viber legacy technology because of its reliance on a centralized system and which will stop working as soon as vc money runs out

Moreover, Viber's adoption is highly concentrated in some specific countries so, depending on where your friends are based, it might not be a viable solution.

I uninstalled Viber when the last friend still using it started using WhatsApp.

Btw, everybody still has email and SMS like in the 90s. For me SMS are a fallback when IM won't work for a given person; email is my primary mean of communication for business. Furthermore I can archive and search email much more than IMs.