For the video and the sheer power of Michael's vocals alone - probably. But then I listen to Man in the Mirror, and Heal the World and will think the same thing. It had the biggest and most epic video for these type of "Conscience songs", but it was the latest too. It is hard to say yes, But If I look at the other "Conscience songs" - dramatic and thoughtful subject matter, pleading and loud vocals and just sheer power we have.
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Thriller - Human nature (Not really environment or war like the other songs - but MJ is questioning why we are that way)
Bad - Man in the mirror (This song covers everything and has the epic video) to me this may be the most powerful song and had MJ had the 1995 video technology back then, he may have had a just as epic video.
Dangerous - This has besides Heal the World, which is powerful, but a bit sugary for my liking with the kiddie choir being trundled out, but the message is nice. I think Will you be there has the most power, yes it's about love and loyalty rather than War, Poverty, Injustice and the Environment in these songs, this song is pure power.
History - Has this song, but also the powerful TDCAU with both of its epic videos and of course the message in the song. So on later reflection probably not. Earth Song is powerful, but it is mainly the video that grabs you.
So in all, I would say Earth song would be the 3rd most powerful behind Man in the Mirror, Will You be there and probably equal with TDCAU. That is not to detract from it, it is still an incredible song and the video is still mind blowing now.