One of the very first anime TV series to get it's own theatrical spinoff was Lupin The Third(or Lupin III). Although this was the third film of anime's favorite thief(the second one to be animated)it is considered to be the best feature ever of him, mostly due to the fact it was directed by Hayao Miyazaki.
Lupin and his partner Jigen begin pulling off a daring casino heist, but realize all the bills they stole are counterfiet. So they decide to go to the source of the funny money, an un-named small European country run by the devious Count Cagliostro. When they arrive, Lupin rescues a teenage girl from some hoods, although she goes with them in order to keep him safe. Lupin learn that she is the Princess Clarisse who is betrothed to the Count, although he's only doing it to find an ancient buried royal treasure. So, Lupin plans to get her from the Count's castle, by calling in Goemon, and then sneaking into the castle disguised as Zenigata who's arrived with his own Interpol squad. Once there, Lupin is sent down into the castle's dungeon, along eventually with Zenigata, although they both escape, but Lupin ends up getting shot. Thinking that Lupin is dead, Cagliostro goes on with his wedding, but Lupin & Co.(along with Fujiko)crash the party. Lupin and Cagliostro have a swashbuckling duel, which leads to the reveal that the lost treasure which is actually a sunken Roman city. Lupin then heads for the hills with the Zenigata Force in hot pursuit.
Cagliostro has been viewed by many as a true anime icon. This was in fact Miyazaki's very first animated movie that he worked on prior to him forming Studio Ghibli. It has exceptionally detailed animation, plus some wonderfully original action sequences, such as elaborate fight in the clocktower, and the best non-Speed Racer car chase of the 70s. There have been two releases of this movie in America. The first was in the 90s on dubbed VHS by Streamline Pictures, and the other was on DVD a few years ago from Manga Entertainment. I'd personally say that the Streamline dub had better casting and dialogue, but the Manga version has a Japanese track and better quality. Oh, and you old school arcade kings might recognize some of the footage from this which was used for the laserdisc video game, Cliff Hanger!