I prefer Black or White. Why? Because everything about it is just perfect. The lyrics, the vocals, the music, the video, the iconic look, the long curls, the era etc etc.
Beat It is solid and has an edgier sound plus its video is also awesome, however it has that... global and "extra" timeless (I said "extra" because Beat It is undoubtedly timeless, but not on the scale of Black or White) factor missing which is present in Black or White. Although I really really love the whole setting - you know, the gangs and everything - it doesn't quite hit the high points of Black or White.
Black or White for me!