A lot of my friends use Whatsapp, but, honestly, out of all the alternatives I find it to be the worst choice. GTalk is very easy to use, but can be unreliable sometimes. I'd say go for Viber, but it happens to be being blocked by my carrier, so it's not an option for me.
I have yet to try GroupMe (by Skype) and I've also seen Beep (by IM+). Has anybody got experiences with these that they could share?