I'll have to go with In Search Of Truth, it captures the dark atmosphere of the album without relating directly to the concept...and I have to give credit to the booklet of RD, many of the images there are perfect for the songs...
Yes, I voted for the ISOT cover, but second probably would of been RD or TDD. I think the dark discovery is a pretty cool cover. I also like how The Inner Circle's cover relates to the concept and Michael Jackson's life too.