Are any of the DC Animated Films inside of a shared universe, or are they all just One-Shots?
I believe that Justice League: The Flashpoint Paradox started a shared continuity that proceeded through quite a few of the movies that have been released since, including the most recent one, The Death of Superman. Most of the movies released before The Flashpoint Paradox were standalones, and some since then have been, too. There are some lists out there as to which ones are included.
Yeah like Eagle said, after Flashpoint they started setting probably around half of the movies they released in a shared continuity. These include:
Justice League: War
Justice League: Throne of Atlantis
Son of Batman
Batman vs Robin
Batman: Bad Blood
Justice League vs Teen Titans
Justice League Dark
Teen Titans: The Judas Contract
Suicide Squad: Hell to Pay
The Death of Superman
A lot of the best movies are completely standalone though, like Crisis on Two Earths, New Frontier, Under the Red Hood, etc.
Many of the pre·Flashpoint films are also loosely tied to the Bruce Timm animated TV shows. For example, Crisis on Two Earths tells a story that is meant to fit after season two of Justice League and before season one of Justice League Unlimited. This movie explains how Wonder Woman got her invisible jet that you see in “For The Man Who Has Everything”. However, there are little differences that makes it not a perfect fit. For example, the movie has a different Green Lantern - Hal Jordan instead of John Stewart.
As others in this thread have already said, the continuity and at style became much more consistent after Flashpoint reset the animated universe.
Speaking of the DCAU, Batman and Harley is made in the style of Batman The Animated series. Also, Batman Mask of the Phantasm, The Mystery of the Batwoman, and Batman Beyond The Return of the Joker (which are all on this service) are all set in the DC Animated Universe alongside Batman TAS, Batman Beyond, Justice League, etc.
Oh and The two Superman/Batman movies are related to each other. And you could probably count Batman: Year One and The Dark Knight Returns parts 1 & 2 as being in the same universe, because the original comics are intended to be.