Best of the Marvel movies by far, probably in my top 3-4 superhero movies along with X2, TDK, Incredibles. Just not sure where at in that order quite yet.
But it is not a perfect movie. The score is instantly forgettable, but had they had some decent hummable character themes it could have been fantastic. Wasted opportunity.
The beginning of the movie drags a bit, getting all the characters to where they need to be. Whedon slips some humor in, which makes it more tolerable, but it still drags.
The Chitauri are weaksauce bad guys. And deeply uninteresting. Basically space pirates, with no piratical charm. The next movie needs someone who can go toe-to-toe with the heavy hitters of Hulk/Thor/Ironman and present a physical threat.
Finally, in the third act the council is way too quick to nuke New York. The Helicarrier should have positioned itself under the portal, and tried and failed to contain the alien invasion(c'mon, that would have been some sweet PEWPEW!). After the Helicarrier crashed or whatever, the council could have then ordered the nuke strike knowing their most advanced conventional military option wasn't going to cut it. It would also have given Nick and Maria some cool stuff to do instead of just watching things on the news.