Skype has some security issues that have yet to be resolved and knowing Microsoft's history with MSN Messenger, issues that will likely never be resolved. Also it is plenty unstable under Linux, sadly. I've found Viber to be easiest, only gotcha is you need it on your phone first before you use it elsewhere. Otherwise it's pretty spot on. It also looks and feels the same across platforms.

I'm a Viber user but I still prefer Skype, it's the easiest platform to talk to people besides Whatsapp, and I think it's the best mobile video platform there's here today is Skype at least on Windows and iOS.