SimpleX may be one of the best, privacy-wise, but until they implement multi-device support with shared history, it's simply a non-starter. Not being able to access a conversation on both my phone and my computer puts a messaging app near the bottom of any usability list.
SimpleX is close to implementing it; the last time I checked, there was a way to link two devices, but it was exceedingly cumbersome - too difficult to ask a non-tech person to work through - and the history syncing didn't work. If they get that worked out, it'll be a strong contender; I only wish it'd been part of the original design and not a tack-on, as I expect it'll consequently be a major source of bugs for the project.