Honestly Discord would be my first option for several reasons:
1. Many of the things Azie said below. There's so much more customisation for Discord servers (which are completely free to make) than Skype's groups. This includes roles (like permission plugins' groups) with custom colours, names, and of course permissions.
2. Honestly I disagree call wise though. The calls are definitely far better quality and I've been able to easily talk to lots of my friends there without issue, and it also supports screensharing and video calls (recent feature). Plus if you're disconnected from a call, unlike Skype, it will automatically reconnect you. So if your internet mucks up like mine, then it won't be as much of a fuss reconnecting.
3. Discord can link your Discord account to other accounts you may have easily, such as Steam, Twitch, Twitter, etc.
4. Support is handled quickly and easily.
5. For some games, including Minecraft itself, Discord can provide an overlay for managing calls and such.
6. Much more but I've lost count. Lol
Any questions just ask, I've used Discord since its Alpha and haven't regretted it.