I’m going to have to go with either Skype or Hangouts, though I realize that likely isn’t what you want to hear.
My boy is in the Navy, based out of Yokosuka, Japan.
We both have iPhone 6’s and use FaceTime. His mom (my ex) has an Android phone so he Skypes with her. I asked him if there was a quality difference and he stated “Not too much”. He then added that he likes FaceTime better because it seems to use less data.
I don’t have much experience with Skype, but a 30 minute VIDEO chat uses about 64MB of data on FaceTime. That’s pretty damned efficient. The audio is superb… basically the best telephony audio you can have, while the video is fairly lo-fi. I swear the audio calls to freakin Japan are clearer than my calls across town to my wife while I’m at work, on T-Mobile’s network
I’m not an Apple fanboy by any stretch, but credit where credit is due… FaceTime kicks alot of ass. If Skype is anywhere close to it in quality, that’s what I’d go for personally.
Edit: You didn’t mention it one way or the other, but Telegram is the bee’s knees for instant messaging. My son and I drop dozens of photos into a 3-way chat between him, myself and his mom. You can go back weeks later and filter those chats by media and look at just the photos, documents, etc. There are clients for it on all mobile platforms, as well as a nifty web interface and even desktop clients. You might find it a very handy tool, if you aren’t already using it.
I can’t recommend it enough.