The best replacement for hangouts is skype.

The best alternatives to hangouts are rated as following:
★★★★★  skype
★★★★★  messaging
★★★★  imessage
★★★★  whatsapp
★★★  talk
★★★  textra
★★★  google voice
★★  stock messaging
★★  gchat
★★  samsung messaging
★  chat
★  messenger
★  google talk
★  samsung sms
★  stock sms
★  gtalk
  bbm

Your alternative for hangouts is not listed? Suggest!
Speaking about hangouts, one probably shouldn't forget about google voice.

Other search terms related to hangouts and their ratings are:
★★★★★ google voice
★★★★ sms
★★★★ google voice sms
You have in mind a companion for hangouts and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of hangouts in all years. Click on graph for more.
hangouts
skype
messaging