I would suggest WhatsApp.
I have used it, and it works really well. Nice and simple. I can't say this first hand, but I have read many times that WhatsApp is really good in bad service situations. I hear people saying they are still available to send messages on 1X data just fine. But if you are just going to be using it from Wifi, I doubt that makes much difference. Maybe the same applies to a bad wifi connection.