Throne of Blood (Kumonosu Jo: "The Castle of the Spider's Web") is ranked by many as the most successful film adaptation of any of Shakespeare's plays. Certainly, it is among an elite few that are not just successful adaptations, but great films in themselves.
The Shakespearean source is explicitly credited in this film, unlike in Ran, and in general Throne of Blood sticks much more closely to the plot of Macbeth than Ran does to King Lear. The main differences include the three witches being replaced with a single demon/witch character, and Washizu's death coming at the hands of his own troops, rather than a MacDuff character.
Most of the main characters of Macbeth are accounted for: Duncan becomes the warlord Tsuzuki, Macbeth and Banquo are his generals Washizu (Toshiro Mifune) and Miki (Minoru Chiaki). Lady Macbeth is Asaji (Isuzu Yamada), Washizu's ghost-like wife, who appears to have stepped straight out of a Noh play. The most memorable scenes from the play are represented also: Banquo's ghost at the banquet, Lady Macbeth's madness etc. What's more, I doubt any director could best Kurosawa's staging of the forest moving towards the castle, which portends Washizu's defeat. The vision of the tops of the pine trees relentlessly advancing through the fog is absolutely chilling.
Perhaps the most memorable scene in the film, however, is totally Kurosawa's addition. Washizu attempts to rally his troops from the battlements of his castle (the eponymous Spider-Web Castle, which he stole from Tsusuki), but their faith in him is shattered on seeing the advance of the forest. One of his archers skewers him with an arrow from below. He doesn't die, and several more arrows are loosed at him. Still alive, he makes his way down from the battlements, as more and more arrows are shot into him. By the time he stumbles to ground level, he must have over a hundred arrows protruding from his back and front, but he still strikes fear into his troops. As he lurches forward in his dying moment, they all draw back in terror. This scene really has to be seen to be believed. A contemporary reviewer found it preposterous, with Washizu resembling a porcupine but still standing, but I've never seen anything like it. Scenes of carnage from movies such as True Romance and Reservoir Dogs pale in comparison.
I think the reason this works so well as a film, compared to other Shakespearean adaptations, is that Kurosawa was freed from the burden of faithfully rendering the text and verse, given that the whole thing was in Japanese. Starting from scratch, he was able to create filmic analogues for the great expressive speeches created by Shakespeare for the stage. For example, Washizu's reaction to his wife's madness is limited to one word, in contrast to Macbeth's lengthy soliloquy: "Fool!" he roars at himself. As a whole, however, the scene creates as much meaning as do Macbeth's lines do on stage.
In short: I can recommend this film without hesitation to anybody with any interest in anything. See it. You must.