YouTube released a list of The 10 Most Viewed Music Videos of 2015.  I'm just as shocked as you are that Adele isn't #1 on this list.  But that's because the other videos had a big head-start.  Here's the Top 10, along with their post-dates and rough view-counts.

10. "Watch Me (Whip/Nae Nae)", Silentó

464 million views.  It was uploaded on June 25th.

9.  "Elastic Heart", Sia

470 million views.  It was uploaded on January 7th.

8.  "Worth It", Fifth Harmony and Kid Ink

481 million views.  It was uploaded on March 28th.

7.  "Hey Mama", David Guetta and Nicki Minaj

506 million views.  It was uploaded on May 19th.

6.  "Hello", Adele

658 million views.  It was uploaded on October 22nd.

5.  "Bad Blood", Taylor Swift and Kendrick Lamar

661 million views.  It was uploaded on May 17th.

4.  "Love Me Like You Do", Ellie Goulding

848 million views.  It was uploaded on January 22nd.

 

3.  "Lean On", Major Lazer and DJ Snake

883 million views.  It was uploaded on March 22nd.

2.  "Sugar", Maroon 5

913 million views.  It was uploaded on January 14th.

1.  "See You Again", Wiz Khalifa and Charlie Puth

1.2 BILLION views.  It was uploaded on April 6th.