The answer to this question has certainly changed over the last couple of years. Here’s how it has changed. Sorry, Apple fans. :-).
As reported by eMarketer:
“According to data released in February 2015 by Opera Mediaworks, Google’s operating system (OS) grew its share of mobile ad impressions worldwide by more than 66% between Q4 2013 and Q4 2014, from 37.7% to 62.7%, leapfrogging iOS to claim first place.”