All in my opinion, but here's how I rank the DC Animated films.
Blue = Amazing
Black = Okay
Red = Awful
Superman: Doomsday
Justice League: The New Frontier
Batman: Gotham Knight
Wonder Woman
Green Lantern: First Flight
Superman/Batman: Public Enemies
Justice League: Crisis on Two Earths
Batman: Under the Red Hood
Superman/Batman: Apocalypse
All-Star Superman
Green Lantern: Emerald Knights
Batman: Year One
Justice League: Doom
Superman vs. The Elite
Batman: The Dark Knight Returns - Part 1
Batman: The Dark Knight Returns - Part 2
Superman: Unbound
Justice League: The Flashpoint Paradox
Justice League: War
Son of Batman
Batman: Assault on Arkham
Justice League: Throne of Atlantis
There has definitely been a dip in quality, but their amazing efforts more than make up for their crummy ones. And they have way more excellent choices in their filmography than poor ones, so I'm a happy camper.