The short answer is that it's the darkest of the 11 films. Star Wars, as an entire franchise, is built on the narrative of the most powerful Jedi (at the time, at least) being seduced by the dark side of the Force. We finally got to see the most important moment in the entire story's narrative. Moreover, it was done in a way that I never thought Lucas would consider. We saw the beheading of Dooku, the brutal slaying of Mace Windu, the Jedi Purge, and the slaughter of the younglings. When Anakin landed on Mustafar to murder the Separatist leaders, we saw the birth of a villain that had haunted so many of our dreams during childhood. It was breathtaking.