Ultimate Avengers and it's sequel are pretty good, but are let down by a portrayal of Hank Pym that's so bad it's not even funny.
New Frontier was pretty good, as was Wonder Woman, GL, and Superman Batman.
If i had to pick one, i'd probably go for New Frontier. It had an amazing cast and quality storytelling. The other DC movies all had their faults, and most Marvel movies aren't worth talking about.
TAS of Bats, Spidey and X-Men still beat any animated movie so far, though, imo...