Who’s the biggest music star of the 21st century? Billboard has answered the question with its newly released rankings, based on performances on the Hot 100 and Album 200 charts from Here are the Top 20: 1. Taylor Swift 2. Drake 3. Rihanna 4. Post Malone 5. Eminem 6. The Weeknd 7. Beyoncé 8. Justin Bieber 9. Bruno Mars 10. Usher 11. Maroon 5 12. Adele 13. Morgan…