From a practical sense, ActivityPub may be the obvious choice as it gives easier interop with the largest federated platforms.
But what else? There are existing platforms built on these protocols, such as movim for xmpp, and another for matrix I forget.
From a technical standpoint, are there any major pros and cons?
If you or anybody else reading should remember the project, I’d be interested in hearing more.
Edit: If it’s this one you’re forgiven for not committing the name to memory…