At the moment I can only recommend Threema and Signal.
Of course there are many other messengers out there. I can not present them all here. That is beyond the purpose of this site.
All other messengers are (as of Janury 2019) either not on par regarding security and/or data protection. Do not waste your time with the wrong messengers. Simple as that.
I just focus on those messengers, who are in line with German law and in line with the GDPR and which do not sell the data of your friends.
This is something I can not stress enough. If you as a messenger user are catching a flew, this is your business. But it is not up to you to decide, whether your friends get a flew too, without asking them beforehand. And this is exactly what you are doing when you install and use WhatsApp.
Do not whitewash it, do not play it down, just because you do not want to have a bad conscience for what you are doing in reality 😉
O.k., so we have only Signal and Threema left on the table.
Signal works on all platforms. Signal is for free (open source) and is very secure. With Threema together the most secure messenger out there. Even Edward Snowden recommends Signal. The colours and userinterface is a little bit outdated (as of 06/2018), but will surely improve with the next updates.