I've only seen the first movie. Minor spoilers follow.
There's no direct gauge, since (at least for the first movie, and Avengers) the heroes are only facing a certain category of opponent who aren't easy to judge.
Certainly the Chitauri, while advanced technologically, weren't high level compared to the Asgardians we're familiar with. IMO neither were the frost giants. A group of five or six Asgardians in a poorly-planned "road trip" beat a bunch of them.
We also need to take into account the level of the other Avengers. IMO, they should all be about the same level. Thor might be an elite, and the Hulk a solo, but the skill levels shouldn't be terribly different. In his suit, Iron Man was a reasonable challenge to Thor. He didn't kick Thor's butt but didn't get his butt completely handed to him either. Samuel L. Jackson might be a fairly high-level minion; he has good mental ability scores and training in a lot of skills, plus probably some minor leadership boost similar to the half-elf ability. That's 4e-speak though.
In 3e terms, the highest level characters, in terms of class levels, would be Black Widow, Captain America and maybe Thor.
Iron Man, Captain America and Hulk have less skill but templates that boost their stats (so their attack bonuses and damage will be pretty good, and very good in Hulk's case).
Iron Man has high Intelligence and Charisma (and low Wisdom), with poor physical stats (scientist and socialite, plus very low Con for the heart condition) relying on his "template" for butt-kicking, while Captain America actually had low physical stats before his template, but has decent Intelligence and good Charisma. His Wisdom is at least decent.
Thor has amazing physical stats and is skilled enough to beat up SHIELD security guards even without his template (good base physical stats and high BAB).
Hawkeye and Black Widow have good all around stats but maybe no template (some comic sources indicate Black Widow has taken a weaker version of the Captain America formula though). She has no dump stats, and her Wisdom is pretty good for playing mind games.
Hulk has lame physical stats but high Intelligence, with the template monstrously raising his Strength and Con and giving him slam attacks, leaping abilities and fast healing.
Hawkeye has decent physical stats plus amazing Dexterity, and at least good Wisdom. He didn't strike me as Mr. Charisma.
And finally, Samuel L. Jackson has decent physical and great mental stats, with lots of levels but no templates, giving him lower CR than the heroes. He might even have levels in an NPC class (aristocrat almost suits him, actually; it gives him decent weapon proficiencies). In 3e terms their CRs shouldn't be terribly far off from each other, but a variance of up to 4 isn't ridiculous.
Oh right, Loki. I don't recall his mystical abilities in the first movie too well. He's a frost giant, though with the Pathfinder "young" template to account for his small size. He can inflict cold damage by touch, perform rituals (spell-like abilities) and likes spamming illusions (a custom spell or spell-like ability) that provides some of the benefits of Mirror Image. He has an essentially permanent Change Self effect on him that even he didn't know about at first. In Avengers, he has a ridiculously powerful staff that can cast Charm or Dominate Person or Monster by touch, lasting a really long time... and Iron Man specifically resists this. IIRC the staff has some sort of ranged attack, and I can't recall how he forced a whole crowd to bow to him. He has the Snatch Arrows feat. He has low physical stats for a frost giant or Asgardian, but very high Int and maybe decent Charisma. He has Bluff but not Diplomacy.
