Generally speaking, the DC animated movies are awesome and the Marvel ones are garbage. Just finished Under the Red Hood and it was awesome.
The Red hood is very very good, top 5 for sure. I also enjoy Doom & Crisis on Two Earths, mob boss Superman and insane (more so) Batman (owlman) are really fun to watch.
Here is a full list i stole from the TV forums (WTF) if you want to fill your back catalog
Released:
2007 Superman: Doomsday Loosely based on The Death of Superman storyline
2008 Justice League: The New Frontier Based on DC: The New Frontier by Darwyn Cooke
2008 Batman: Gotham Knight Collection of six shorts. Set between the events of Batman Begins and The Dark Knight.
2009 Wonder Woman
2009 Tales of the Black Freighter Related to Watchmen film, an animated adaptation of the comic Tales of the Black Freighter within the story
2009 Green Lantern: First Flight
2009 Superman/Batman: Public Enemies Based on Superman/Batman: Public Enemies storyline by Jeph Loeb
2010 Justice League: Crisis on Two Earths Adapted from an unused movie plot meant to bridge Justice League and Justice League Unlimited. Loosely based on JLA: Earth 2 series by Grant Morrison
2010 Batman: Under the Red Hood Loosely based on the Batman: Under the Hood storyline by Judd Winick
2010 Superman/Batman: Apocalypse Direct sequel of Superman/Batman: Public Enemies release. Based on Superman/Batman: The Supergirl from Krypton storyline by Jeph Loeb
2010 DC Showcase Original Shorts Collection Compilation of short films under DC Showcase label
2011 All-Star Superman Based on All-Star Superman series by Grant Morrison
2011 Green Lantern: Emerald Knights Collection of six shorts. Coincided with the release of the Green Lantern film
2011 Batman: Year One Based on the Batman: Year One storyline by Frank Miller
2012 Justice League: Doom Loosely based on the JLA: Tower of Babel storyline by Mark Waid.
2012 Superman vs. The Elite Likely based on the What's So Funny About Truth, Justice & the American Way? storyline by Joe Kelly.
2012 The Dark Knight Returns - Part 1 Based on the The Dark Knight Returns storyline by Frank Miller
2013 The Dark Knight Returns - Part 2 Based on the The Dark Knight Returns storyline by Frank Miller
2013 Superman: Unbound Based on the Superman: Brainiac storyline by Geoff Johns and Gary Frank
2013 Justice League: Flashpoint Based on the Flashpoint storyline by Geoff Johns and Andy Kubert