Close,very close,Superman wins it out for me by a hair.
Superman. He's about as American as it gets.
Pfft. Don't make me laugh. To be American is to be a citizen of America. In this regard, Superman and Captain America were equal, until Superman renounced his citizenship, because Superman belongs to everyone, even tyrants, warlords and slaughterers the world over. Steve Rogers is proud to be American, while Superman is embarrassed to be associated with that country. Steve was "American" enough to volunteer for military service, not as an invulnerable demi-god, but frail, 90-pound weakling Steve. When he finally got his wish, he served his country with distinction, to the fullest extent of his newly-superhuman ability. Talk to me when Superman puts aside his messiah complex long enough to topple dictators in the name of freedom as Cap has (which he can do easily, heck without killing anyone, whereas Cap had to win victory through blood, sweat and tears).
They say Superman's brave, but he's bulletproof. Cap won respect not by being a "good guy" magnified by incredible powers, but an inspiring leader. It is not powers, but faith and conviction that makes Steve Rogers a god among men.
Freedom ain't free. It's easy to forget that when your powers insulate you from reality, when you split your time between Antarctica and space, and can fly away and leave the world behind in a heartbeat.
Wanting to topple dictators is why he renounced his citizenship in the first place,come back when the U.S.A spends as much effort toppling the House of Saud as Assad,heck even that is being done at the behest of the Sauids' never mind the potential long term implications,you know who are fighting alongside the U.S.A to overthrow Assad? the ISIS and Al-Qaeda. They love dictators who serve their interests and hate those who dont.