I don't consider it one of his best, but it's definitely a must-see. My main problems have to do with story and structure. It's loosely based on Shakespeare's King Lear, which I must admit, I've never read or seen performed. The plot is a little dense and hard to follow at times, regarding an aging warlord who leaves his kingdom to his eldest son and inadvertently sets off a power-struggle. Part of the problem is that Kurosawa does little to distinguish the warlord's sons, except to color code them so that the viewers can tell their armies apart. Otherwise, it's hard to tell who's conspiring against who and for what reasons. The focus is on the warlord for much of the film, but in the middle numerous subplots emerge and distract from the main narrative, and then in the finale the warlord seems to drift into the background. So it's a little hard to get too invested. Many scenes are interesting in and of themselves, but the characters and relationships never seem as well developed as they should be because of the shifting focus.
The reason this is a must see is because Kurosawa brings the spectacle. Besides the constant presence of bright primary colors and lots of precise, eye-drawing framing and camera movement, Kurosawa stages some epic battle scenes that will make your eyes bug out. An attack on the warlord's castle about an hour into the film is especially powerful. Kurosawa drops out the dialogue and sound effects and fills the soundtrack with haunting, mournful music, as a seemingly infinite number of men are cut down by swords and filled with arrows, and the entire stronghold drowns in blood.