One Piece Movie 06: Omatsuri Danshaku to Himitsu no Shima

Fun group of neatly designed characters with good chemistry. Originated from TV series and directed by high-profile directors (Miyazaki and Hosoda). Castle of Cagliostro actually inspired Mamoru Hosoda when he was in 6th grade, hence why his own movie shares so many similarities with it. 

Same director. Both films in two of Toei's biggest properties. Similar artistic style and animation. Both visually outstanding in comparison to most other works from each of their time periods, and great as standalone movies too. Both enjoyable and worth a watch, especially for fans of either series. 

Both movies are works of director Mamoru Hosoda and share a similar atmosphere as they both start in a light hearted and funny manner and become more serious in a believable way later. Although they were not produced at the same studio the movies have similar detail reduced artwork and really splendid animation.