Well, it really depends on where you live and which services your friends/contacts really use. Whatsapp is super popular in many countries, but if you were in China, for instance, Line would be your weapon of choice. There's also Facebook's Messenger, BBM, Samsung's ChatON, and of course, an Android favorite, Google Hangouts.
Facebook Messenger is your best alternative as practically everyone uses Facebook.Telegram,LINE,hike are also good messaging apps;but then again,use the one which is popular in your area