Most of the DC animated films can be in the same continuity...
I'm referring of course to the 'original animated films' line.
I know that for the most part, they are supposed to be stand-alone, but I think the majority of them can fit into the same universe/continuity. Moreover, I think if you arrange them in chronological order, they make up a coherent timeline that spans the DC Universe from the early days to the present...and by watching the films in that order, you can pretty much witness the evolution of the DC Universe and its heroes.
This is how I feel the timeline would go for the DC 'animated film' continuity-
Batman: Year One
Wonder Woman
Green Lantern: First Flight
Superman: Doomsday
Superman/Batman: Public Enemies
Justice League: Crisis on Two Earths
Batman: Under the Red Hood
Superman/Batman: Apocalypse
Green Lantern: Emerald Knights
Justice League Doom
Superman vs. the Elite
Superman Unbound
Justice League: The Flashpoint Paradox (?)
Now there are obviously a lot of the other films which simply cannot fit in. 'Batman: Gotham Knights' is meant to be set in the Nolanverse. 'All Star Superman', 'Justice League: The New Frontier' and 'Batman: The Dark Knight Returns' are all essentially 'Elseworlds' stories set in their own separate continuities. But that apart I think all the DC movies are meant to be in the same canon.