There are only a few free apps that you could trust nowadays. Most of the free apps have so many ads that it becomes an annoying task to use them with ads and game advertisements literally being rubbed on your face. Also, some are downright scams. But I would have to say that WhatsApp is my favorite.
As far as paid apps are concerned, there is only one app that I actually paid for - a monitoring app called Xnspy. I use this app to keep on my teens from work. It has some pretty cool features (text messages and call monitoring, surround recording, GPS location tracking, and lot more). I can know where they are and what are they doing from miles away.