Castle of Cagliostro is a very good Miyazaki film, a great anime film, but NOT a very good Lupin III feature. Lupin's too out of character and the other regulars barely get any screen time.
The best Lupin features are Secret of Mamo, Dead or Alive, Memories of the Walther P-38, To Hell with Nostradamus, Mystery of the Hemingway Papers, $1 Money Wars, Fuma Conspiracy, Alcatraz Connection, and First Contact.
Many of the others are mediocre to horrible. Your mileage may vary. For instance, the feature Nabeshin directed, Fujiko's Unlucky Days, was the worst piece of shit I've ever watched.
The greatest age of Lupin was the succession of awesome that was $1 Money Wars, Alcatraz Connection, and First Contact. Those were the best.