Matrix 100%

It supports e2ee, which is a plus for privacy
You can switch out your instance to a different one and still communicate with others (federation), this can be for speed since the default matrix.org is a bit slow at times, or for better privacy.
All the tech communities are currently there which can be a plus or a negative for you
It supports bridging to a tonne of different platforms. Want to use matrix, but you still want people from discord to be able to communicate to the server? Setup a discord <--> matrix bridge, which is literally as simple as just inviting a bot on matrix, and the same bot on discord. Then users can choose to use discord or matrix when joining the server, messages will be transmitted to both. (Theres also bridges for telegram, whatsapp, signal, IRC, and a bunch of other messaging platforms)

In terms of a client i would highly recommend cinny as a webclient, or fluffychat as a linux/mobile client. Though there's also other clients for matrix, schildichat, nekochat, fractal, gotktrix, mirage, etc.

I do still keep a discord account though, since some servers i'm in haven't setup a bridge.