I've exclusively used Skype for video chat with family (who happened to be in a really infrastructure-poor part of the world) for months - it was by far the only thing that worked, pixelated low-res video notwithstanding. Most of the other clients, e.g. line, gave-up within 30 seconds with errors like "connection is unstable". Getting Skype to connect is a major pain but once it gets going, even though the video sucks, it isn't choppy and the connection doesn't keep dropping.

Also, I never faced any ads with Skype (not sure if that was due to $1.20 balance in my Skype Credit account). The aforementioned calls were over wifi on my side and 2G(?) connections on recipients side, so free.