The problem I've observed with XMPP as an outsider is the lack of a standard. Each server or client has its own supported features and I'm not sure which one to choose.

That's a valid concern, but I wouldn't call it a problem. There are practically 2 types of clients/servers: the ones which are maintained, and which work absolutely fine and well together, and the rest, the unmaintained/abandoned part of the ecosystem.

And with the protocol being so stable and backwards/forwards compatible in large parts, those unmaintained clients will just work, just not with the latest and greatest features (XMPP has the machinery to let clients and servers advertise about their supported features so the experience is at least cohesive).

Which client would you recommend?

Depends on which platform you are on and the type of usage. You should be able to pick one as advertised on https://joinjabber.org , that should keep you away from the fringe/unmaintained stuff. Personally I use gajim and monocles.