Cap was the last one standing against Thanos because Thanos thought of him as a weaker opponent as the rest of the assembled heroes. Cap was surrounding by the unconscious bodies of his comrades, guys like Thor and The Hulk, yet he still charged at Thanos like he would have done something. Thanos grabbed him by the throat and asked him why he persisted in this futility and Cap responds in one of the greatest lines I have evey heard in comics, " While one of us still stands you can't claim victory." This guy is literally looking in the face of death and has the balls to say that! This is only one of the reasons I choose him.
Cap leading the JLA and Avengers in JLA/Avengers #4 and leading the undead army of heroes in Universe X shows that this guy is so good that the most powerful heroes in 2 universes look to him for example and guidance. When the 2 teams were decided on a plan of action and they needed a leader Superman, the leader of the JLA and EVERYONE'S hero, turned to Cap without hesitation and asked him to lead. That is a big deal in my opinion. Captain Marvel's plan in Universe X depended upon the undead heroes to believe in themselves and team up. They all had trouble believing in themselves since they didn't think they were dead. If the above if the greatest line, then this has to be the greatest scene: Mephisto looks out at the barren afterlife and tells Captain Marvel that he has lost, then all of a sudden we see the entire population of undead heroes assembled ready to fight. Mephisto looks shockingly to see how this is possible and sees Captain America raise his shield. I got goose bumps, seriously.
Cap repesents everything a hero should at least aspire to be. He projects a feeling of patriotism but not for the USA but for the world. He is a living symbol of humanity, not just the USA. I think the best way to describe what Cap means to the Marvel universe was best said by Falcon in an issue of the Avengers last year, "Compared to Cap, we're all sidekicks."