I must disagree completely. Mussorgsky's Tableaux is perfect on the piano, and I must admit I dislike all transcriptions I know of this work, from Ravel's orchestration to Emerson/Lake/Palmer's whatever.
By the way, it makes me think that this is one - perhaps THE one - of the most tortured pieces ever written, because pianists - who should play it - seems to respect the score too much, and everybody else feels free to do anything. Just listen to Kazuhito Yamashita murdering...err...playing it on the guitar...well, guitarists love his version, but they know very little about non guitar music. And about music in general.
That said, and excusing me for the outburst, I think one of the most incredible transcriptions I know is Prokofiev's Romeo and Juliet ballet to his opus 75. He creates a remarkable work of neoclassicism, painting the music in black and white. This creates such an effect...just listen to the last movement. Or even better, listen to the whole series and pay attention to the last movement. It is beyond my words to describe.
Another interesting examples are the transcriptions made by Liszt of opera, lieder. If you want something that puts another light on the original, then it comes to my mind.
Best regards,
Jay.