I just read a review that declares Ultimate Avengers the "Best Animated Superhero Movie of All-Time!" I'll go one better. It's the best animated action movie of all-time, beating big boys like Titan AE, Transformers and The Iron Giant. Most sane people agree that it certainly beats any single episode of Justice League, X-Men Evolution, Batman or Spider-Man. Some want to compare it to these TV series and their entire run but that's apples and oranges. If Ultimate Avengers spins off in to a series, then there can be discussion.
As it stands now, Marvel's first animation effort set the bar quite high, not only for its future endeavors but for other companies and other studios. Ultimate Avengers proves that animated features don't have to talk down to a presumably child audience. You can set up characters and develop a story just like a live action film. The bonus being that once it's time for action, animation offers no limits. Ultimate Avengers proved this in spades. The alien battle and Hulk brawl were intense and spectacular. Setting them up, though, was what elevates this film past the other movies mentioned above. Marvel's live action movie experience bringing the likes of Spider-Man, X-Men and even Daredevil to the screen must have given them the edge. They've brought a remarkable animated world to life with Ultimate Avengers.