Captain America is to the Marvel multiverse what Superman is to the DC multiverse. Though he's far from the most powerful hero, Steve Rogers is the embodiment of Good in the Marvel Universe. Especially when he was the director of S.H.I.E.L.D..
Spider-Man fulfills this when he can. If Cap is considered the Soul of the Marvel Universe's hero community, then Peter Parker is definitely the Heart of the heroes. He manages to unite even the most cynical of heroes and loathsome villains together or brings out the best in them with his unceasing optimism and true everyman nature.
Speaking of Spider-Man, he and his fellow spider-related heroes owe their existence to the Great Weaver, the totemic entity that turns certain people into its champions, and has chosen Peter Parker as its new avatar.
And in terms of sheer power and authority, there's Thor's pop, the Father of All, Odin. He rarely interferes with Earth nowadays, but is clearly this for his native Asgard. (At least in older stories.)
Spider-Man: Life Story: Captain America is shaping up to be this. In Issue #2, Reed Richards considers him to be the Only Sane Man doing the right thing in the Vietnam War, "Going rogue, saving lives on both sides".
Thunderbolts: Jolt was the Big Good to Baron Zemo's Big Bad in the first few years. In some ways Hawkeye was the Big Good after he joined.
X-Men: Professor Xavier is The Leader of the X-Men, principal of the Xavier Institute for Gifted Youngsters, and the standard-bearer of the campaign for mutants and muggles to co-exist peacefully. Even though he doesn't play a direct leader role in all of the Marvel Universe's mutant teams, The 'Verse's various teams of mutant heroes (the X-Men, X-Factor, X-Force, Generation X, etc.) have occasionally been collectively referred to as "The Disciples of Xavier".
In Bendis' Uncanny X-Men, most mutants consider Cyclops this. Though the truthis more complicated, there's no denying his political clout, or his devotion to protecting the species.
Ultimate Marvel: S.H.I.E.L.D. in general, Nick Fury in particular. S.H.I.E.L.D. is the Government Agency of Fiction, oversees all superhuman activities (heroes and villains alike), takes an active role in detaining superhuman criminals (they do not simply sit and wait while Holding Out for a Hero), and have effective Tailor-Made Prisons for superhuman criminals, which are almost never made of cardboard. In addition to that, Fury serves as a Parental Substitute for Peter Parker, they paid the Hero Insurance of the X-Men for some time, they were indirectly the chiefs of the Fantastic Four (as they command the soldiers that command the Baxter Building), and the direct chiefs of The Ultimates. And, when S.H.I.E.L.D. was temporarily out of action during the civil war, Nick Fury served this role personally.
Ultimate Marvel Team-Up: Spider-Man can not deal with the Hulk all by himself, just delay him a bit. That work is for Ross and his soldiers.
X-Men: The Last Stand: Professor X is this most of the time, but it's deconstructed to some degree in this movie. He's called out about putting up mental blocks in Jean Grey's mind in-order to suppress the split persona that called itself the Phoenix. Needlessly to say, Xavier had more or less I Did What I Had to Do in-order to suppress the Phoenix.
X-Men: First Class: Charles Xavier is the leader of the heroic team that eventually defeats the Big Bad. He brought them together, trained them and inspired them to be valiant when faced with great peril.
X-Men: Days of Future Past: The elderly Professor X and Magneto share the leadership role among the remaining X-Men and the Free Mutants. Wolverine's mission is to guide Xavier's younger self towards the process of growing into this by accepting both his powers and his duty to his fellow mutants.
X-Men: Apocalypse: Professor X; the good vs. evil conflict chiefly revolves around him and the Big Bad En Sabah Nur, who are each other's Foil, and the latter is also the former's Evil Counterpart and Shadow Archetype. Charles, despite being relatively feeble as an individual mutant compared to his god-like foe, is the better leader because he's the epitome of Machiavelli Was Wrong, and that's why the X-Men triumph whereas the Four Horsemen crumble as a team.
Live-Action TV
Legion: According to Melanie Bird, her husband, Oliver Anthony Bird, founded Summerland (with Cary), meaning he was interested in understanding the Fantastic Science of mutants. While his first appearance doesn't seem to match his wife's rosy description of him, Oliver is the first person who is aware of TDWYE within David's mind without being told about it or seeing it firsthand.
Video Games
Marvel Ultimate Alliance 3: The Black Order has Nick Fury, a super-spy and natural-born leader who sits at the head of the global defense organization "SHIELD". He doesn’t hesitate to jump into the fray and show those who threaten Earth why he commands the respect of Earth's greatest heroes.
Captain America is the long-time leader of the Avengers as well as the leader of the heroes fighting against Ultron.
Although he has long since passed on, Doctor Light resumes to aid Mega Man X and his allies through special capsules containing his memories, personality, and holographic likeness.
Midnight Suns has Caretaker, who leads the others in the battle against Lilith, aiding the Midnight Suns in their battle with her advice.
Spider-Man: Shattered Dimensions: Madame Web qualifies as the most important good character because she asks for the other Spider-Men's help after Amazing broke the tablet, and gives advice and helpful abilities to each of them.
Western Animation
In Spider-Man: The Animated Series, Captain America is a WWII super soldier, as well as the leader and most important member of the American Six.
In The Super Hero Squad Show, Captain America/Steve Rogers is consistently shown as the most important hero in the show, being S.H.I.E.L.D.'s best agent, both advisor and ally to the the Super Hero Squad.
Wolverine and the X-Men: As always, Professor X is the leader of the X-Men, principal of the Xavier Institute for Gifted Youngsters, and the standard-bearer of the campaign for mutants and Muggles to co-exist peacefully.
Much like in the comics, Charles Xavier is a highly intelligent and powerful telepath, the leader and founder of the X-Men, and the founder of the Xavier School for Gifted Youngsters.
In X-Men '97, Magneto tries to occupy for this role by making Charles' dream a reality, but he's still pretty aggressive and temperamental, even if not without cause.
