The first few Lupin Part One episodes are fairly close to the manga in content and somewhat darker but Miyazaki and some other future Ghibli people take over and tone it down considerably, and there's some kind of proto-Cagliostro plotted episodes.
The popular Part II/red Jacket series seems to balance things a bit more content-wise, some episodes are more serious while others are more goofy, almost Hanna Barbera-style comedy. The first film, Mystery of Mamo, is fairly closer to the manga's style, especially the character designs. It also seems to be going for a fairly surreal tone at times, in a few ways similar to Beautiful Dreamer-it's very different from Castle's almost fairy tale feel but it has it's creative moments.