Billie Jean.
But honestly, I got about 40 "favourite" MJ tracks. You should have said top 10 or sum.
My top 10:
1. Billie Jean
2. You Are Not Alone
3. Beat It
4. Thriller
5. Earth Song
6. You Rock My World
7. Bad
8. Another Part of Me
9. Leave Me Alone
10. ....I can't finish this list because I would have to leave off too many favourites