Apple selected the Periscope app, owned by Twitter, while the most popular app of 2015 in Google Play store is Google Photo. The selection of the best apps of 2015 includes apps for photographs, communication and entertainment. The best aps of 2015, selected by Apple and Google, are apps which are a must have of 2015. 

5 apps of the year by Apple:
Periscope
Workflow
Enlight
Instagram
Robinhood

5 apps of the year by Google play:
Google Photo
Skype
Microsoft Word
Dubsmash
Retrica

There are also only 3 apps Apple and Google both agreed were the best of 2015:

Robinhood:
Robinhood is an app built around one promise: no-fee stock trading. The app makes trading cheap, intuitive, and mobile. The beautiful interface won an Apple Design Award.

Jet:
Jet is a new kind of app — a smart shopping platform that finds ways to turn built-in costs into opportunities to save you money.

HBO Now:
the new standalone streaming service that gives you all of HBO®—every season, every episode of HBO's original series—plus more of the biggest and latest movies than any other streaming service. All you need is the internet.