Probably OTW, Bad and Dangerous, closely followed by Thriller. I do not listen to HIStory and BOTDF as often: they're very good albums but less accessible for on-the-go listening. I never listen to Invincible anymore, apart from the very occasional spin I give Break Of Dawn.
Well thanks to my depression and anxiety attacks. I can only handle listening to him 2 days out of the year. So I really can't really say what my most played album is now. But back when we still had him. My HIStory, Bad, and Dangerous albums were definitely my most played. Most especially Dangerous because of Who Is It. I did have quite the obsession for that song. I have so many different versions of that song. More than I do with any of his other songs.
Even though BAD is provably my favorite album, the one I listen to the most is definitely THRILLER. I've read some of the previous comments and Dangerous seems to get most of the love ;_; poor Thriller