I'm pretty sure if I ask this question in my economics class, then the answer will be Spotify! Indeed a quick show of hands a few days ago revealed that, not only did most students use Spotify regularly, the majority were now paid-up subscribers, joy to the ears of Spotify founder Daniel Elk!

But perhaps Spotify's near 40 million user base looks small compared to the number of listeners to music available on You Tube? I was really interested to come across this chart from Statista.

You will find more statistics at Statista

According to data released by the International Federation of the Phonographic Industry (IFPI), 82 percent of YouTube users utilize the platform to listen to music.
YouTube pays lower licensing fees to the music industry than services such as Spotify because it classifies itself as a hosting platform rather than a content distributing platform and a platform for aspiring/emerging musicians and bands to promote their music.

Key data for Spotify

Key data for You Tube