I've always liked Captain America. Captain America was conceived in 1940, long before the USA entered the war, and represents the author Joe Simon's belief that what America should be doing is getting involved and punching Hitler in the face. You can't argue with a character genesis like that.
The film starts just after America's entry into the war, and we see asthmatic pipsqueak Steve Rogers serially trying and failing to get himself enlisted. We also see him ineffectually standing up to bullies a lot. Lots of heart, but no muscle to back it up. On his latest attempt to get enlisted, he is spotted by Dr Erskine, a German scientist who has a serum which gives people super powers. He was forced to use it once before on Nazi Bad Guy Johann Schmidt, but he's now defected to the allies, and is almost completely sure he's worked out the slight dermatological side effects now.
After an almost complete success in the first test (i.e. Steve Rogers gets Super Powers - Good; German spy assassinates Erskine, and all samples of the serum are lost - Bad) America is left with having only one super soldier. Who ends up somehow becoming a celebrity in a traveling USO show, under the name Captain America.
This isn't enough for Rogers, he wants to be fighting the bad guys on the front lines, not selling war bonds, so at the first sign of trouble he's off on an unsanctioned solo mission to rescue some captured allied soldiers from the clutches of evil Nazi science think-tank Hydra, commanded by Johann Schmidt. Schmidt, meanwhile, has managed to nick an awesome power source that used to belong to Thor's dad, and is using it to build weapons which will allow him to win the war at a stroke. And someone's really got to do something about that.
Basically, this is all dumb, good-hearted nonsense. Captain America as a proper good guy, no side to him, no dark edginess, just basically a guy who's going to stand up for what's right, no matter what. Schmidt, on the other hand, is a proper creepy Nazi megalomaniac, (it's Hugo Weaving! Of course he is! He made Elrond seem like a creepy Nazi megalomaniac!), and the whole thing starts off as a stirring tale of this brave kid taking a huge risk in order to be able to stand up to assholes like Schmidt, then it all kicks off in an epic maelstrom of punching things and shooting them.
It's easily as good as any of the other modern Marvel films; taken singly, I think any one of them is great. Taken as a body of films which are going to come together as The Avengers next year? My only problem is *next year*? Why not *now*?