Dark Elves are best. The other two are close to joint worst. At least the Elves did some decent damage in their initial assault, and they actually went up against all the defenses of Asgard. Plus, if they get Kurse, it tips things massively in their favour in terms of badassery. The Chitauri never impressed me as an overall fighting force, and we haven't really seen that much of the Kree either, but it has been mediocre so far.
In the end though, the Chitauri invasion was essentially stopped by one nuke, so they win the jobber trophy IMO.