Matrix because it seems like the most logical choice - largest platform that’s federated/decentralised and has end-to-end encryption.

Personally I’ve had consistent problems with messages not un-encrypting in Matrix, requiring frequent re-sending of messages. I’m also not a fan of how much Metadata is shared across the Matrix network even with encryption, nor am I fan of the group who developed and funded it, or the willingness of the Matrix/Element team to sell their services to law enforcement.

Movim has all of the same features as Matrix without those downsides, if you’d like to give that a try instead.