If every contact you want to talk to is on Skype already, and you only want to talk to people using whatever facilities Skype offers, then Skype is the one for you.
If you only need to talk to people on mobile handsets who already have WhatsApp, then that’s perfect for you.
Google Hangouts actually still has XMPP under the hood in places, but similar rules apply.
The interesting question is where you need something more. Maybe your contacts all want to run their own server. Maybe you need better security. Maybe you need to exchange arbitrary data. Maybe you’re not happy with one company knowing all your secrets.