I'm fairly certain Gajim supports most of the standard XMPP features one might expect from a client [0]. I use it daily. Granted Jingle audio/video is not working in a cross-platform fashion but I'm fairly certain file transfer through Jingle does. HTTP file transfer is also fairly straightforward if your server supports it. My problem with Gajim is that it's not a turnkey solution. You need to adjust settings, maybe use a BOSH proxy if you're behind restrictive firewalls, manually enable OMEMO and deal with trusting and enabling keys. For instance I occasionally miss OMEMO encrypted messages (as in they cannot be decrypted) send from Gajim on a specific device. On the other hand Conversations does a much better job at hiding all this manual labour from the end user to the point that I can tell anyone: "just install Conversations; login into that server; done".

Pidgin is quite lacking as far as Carbons and OMEMO is concerned. Last time I checked you need to install custom plugins, issue text commands to configure the plugin, issue different commands to trust/distrust OMEMO keys etc. It's a lot of work which is probably partly due to the fact the libpurple tries to handle a lot of different protocols using a more-or-less transparent API. It does a fairly good job with most protocols (including proprietary ones through plugins; like hangouts [1]) but unfortunately its XMPP support is not really up to modern standards without effort (much more so than Gajim).

Dino is good and I think they solved some OMEMO compatibility issues they had. But it's Linux only. This is not necessarily bad since they can do much better work by focusing on a specific platform, both functionality- and aesthetics-wise. It's too bad there isn't anything comparable on Windows/Mac.

[0]: https://dev.gajim.org/gajim/gajim/wikis/help/GajimXEPSupport

[1]: https://bitbucket.org/EionRobb/purple-hangouts/wiki/Home