Gregory's ServerDelta Chat grants any e-mail service involved full access to the respective chat's metadata – possibly including GAFAM, depending on your contacts. Not recommended at all.
Wire doesn't prevent metadata either and is also partly based in the US, meaning they are likely forced to collaborate with NSA etc. Don't use it!
XMPP and Matrix also don't prevent metadata, so you have to trust all your contacts' servers. Avoid if possible.
@BreachCat Threema protects your metadata from third parties and collects as little as possible. Decent choice.
Signal's metadata protection is a bit better; it supports Sealed Sender and PFS, but also requires your phone number. Good choice.
Briar is the best option in terms of security and metadata protection, since it's decentralised, meaning that no server is involved at all. Though, that requires both sender and recipient to be online at the same time.