For Privacy, WhatsApp was to be avoided for years - I used it & still use it with a few people because some friends don’t see this as an issue. Now they claim they have E2EE but can we trust Mark Z. ? At least, they collect metadata or other information.
I used Signal for the past years but it has also some issues :
you loose your database/history chats when migrating from IOS to Android / migration is ok when just using the same Mobile Operating System
it lacks some features. For instance, you can’t save all photos/attachments in IOS. You’ve to save them one by one - Reddit - Dive into anything
I like Keybase; it does encrypted chats and has groups like Slack, but I mainly like the profile verification system and small private file storage. I used it on Windows, Android, iOS, and Linux (RPM installed as-is on Fedora)
I lost interest in Signal back when they wanted to defend not having another option except Google’s GCM. They eventually caved and did Websocket, but I didn’t like their stance prior to that. I recall there being something odd about Signal not being on F-Droid too.
I use Telegram, easy to install from RPM, auto-updates itself on launch. Allows large file transfers, and as an added bonus is even complying with recent law-enforcement efforts
You do have to register an account on a E.164 enabled device first though, which is a bit outdated but it does work well all things considered.