Okay so I've not watched many that aren't tied to existing series, so sorry but you might need to do some homework before you can watch them.
Okay so first Fate/Stay Night Heaven Feel - now this is perhaps not the best entry point to the series - but this is peak animation, so when you get here it is well worth it.
So the first is Presage Flower, followed by Lost Butterfly and ending with Spring Song.
Moving on from Fate, I'd recommend Madoka Magica Rebellion.
This is a pretty awesome film, it is a sequel either to the series or two the movie adaptation of the series (beginning and eternal). So you could watch them as three movies, or a 12 episode anime with a sequel film.
Okay something a little older and a standalone to boot, Royal Space Force: The Wings of Honneamise, I found it a pretty engaging movie with obviosuly awesome animation and backgrounds as you can expect from 80s Anime.
And finally the end to the epic saga, this probably won't be the only recommend for this movie as @MarathonGuy1337 is a pretty big Gundam guy but if you ever watch the Gundam trilogy or the first series, check out Chars Counter Attack for at least the pretty visuals and the epic sendoff to the biggest rivalry in anime history.