Interestingly enough, my vote goes 110% to Who Is It.
To support my pick, I have to stress that we all know that it's one of Michael's many magnum opuses in terms of composition and vocal delivery. It's a love song turned into tragedy; Michael lost his love to another man that very well could be his best friend, his brother, a colleague, anyone. He reminisces the memories he still has, demanding to know who he lost his lover to and what that man has that Michael doesn't. The composition is phenomenal, the vocals are intense and the ending - Michael's beat boxing stacked over actual percussive elements - is one of the better endings Michael's created.
In terms of actual love songs, my vote goes to Beautiful Girl. For some reason I don't listen to it as often as I would imagine I should, but it's still one of Michael's better compositions. The vocals, the composition, the playful "da da da's" replacing the lyrics he didn't write... it's a big shame Michael never finished it. (Or if rumors that he did are true, it's a shame we've yet to hear it.)
