The title may throw you off, but Princess Mononoke is good.
The usuals like Ghost in the Shell, Ninja Scroll, Akira, Vampire Hunter D, Batle Angel, etc.
The Cowboy Bebop movie is well done and can serve as a stand alone, you don't have to watch the series.
If you watch Bleach or Full Metal Alchemist, the first movie for Bleach was well done I thought, as is FMA's Conqueror's of Shambala.
The Grave of Fireflies (something like that), about children in Japan trying to survive during WW2, and hiding in the mountains during bombings while starving, was sad but a good movie.
Perfect Blue is good, but a bit of a mind **** at certain parts.
The Samurai X movies are good watches (I believe they can be watched as stand alones).
There are more, but I'm a bit brain dead atm lol. I do get caught up in anime series too much, there are several movies I've wanted to watch but keep putting off. Wings of Honneamise and Metropolis both look good, but have yet to watch them.