I'm not sure if this is a failure of YouTube advertising it, joke intended, but YouTube Premium is the best value monthly subscription that I have. I just recently upgraded to the family plan at $17.99/month so I could add two of my friends.
That's ~$6 per month to enjoy YouTube completely ad free, allows you to download YouTube videos to your phone for offline viewing, allows background play, _and_ gets you YouTube Music Premium(I still am using Google Play Music, but this is basically a "free" Spotify subscription) as well.
Its such a good value for me and I think it'd be a great value to a lot of other people as well but every time I mention it I feel like people have never heard of it.