Dick Grayson.DC Comics' official position is that he's the most beloved superhero in the community (yes, even moreso than Superman) and that almost any member of it would instantly drop what they were doing to help if he were in trouble. He's also been shown to get along famously with Superman since both characters were created.
Tim Drake is also a nice guy, especially early in Robin (1993). He took care of his girlfriend who was pregnant with her ex-boyfriend's child, thinks of others before himself and is good at babysitting kids. In the new DCU, he was also called a nice guy by Wonder Girl (although he's considerably less likable in the new continuity).
Alfred Pennyworth is loved by pretty much all of the Batfamily and is often considered The Heart, and for very good reason. Unfailingly polite, kind, and loyal, Alfred is always there for Bruce, the family, and their friends, providing help whenever he can, whether it's in the field, or out.
In Fables, Boy Blue is possibly the nicest character in the series, being eternally friendly, polite, hard-working and caring. However, Beware the Nice Ones is in full effect; in his past he was a brave soldier, but he deliberately chose to be a boring office clerk because he simply didn't like adventure and fighting. When circumstances demand his return to action, however, he reminds everyone just what he can do, and even when he's not in action he's not afraid to call people out if they're behaving stupidly. The problems of the Nice Guy character in a romance arc actually get discussed in-story; Rose Red was definitely attracted to him after his adventures, but the attraction wanes when she realises he doesn't actually want to be an exciting person and was quite content to be an unpretentious nice guy, which didn't mesh well with her own preferences for danger and excitement.
The Frog Prince Ambrose is another example, though in a slightly different way; Ambrose at first gave off the Dumb Is Good vibe, while Blue's intelligence, courage and skill were known quite early on in the series, making his niceness more of a deliberate choice. However, it's later discovered that Ambrose takes it all the way up to Incorruptible Pure Pureness levels, and the Messianic Archetype powers he later gets are dependent on him maintaining this. He is best friends with Boy Blue, who points out that Ambrose was the only fable who had absolutely no crimes to be absolved of under the amnesty.
And of course, The Flash. No matter which one, The Flash will be this. Golden Age Jay Garrick, Silver Age Barry Allen, Bronze Age Wally West (though he bounced between this, Jerk with a Heart of Gold and Chivalrous Pervert), Audience-Alienating Era Bart Allen, all of them are consistently great guys who can and will become the best friend and surrogate brother of anyone they're with.
The original Green Arrow is a particularly rough Jerk with a Heart of Gold, but his son, Connor Hawke, is one of the foremost examples of this trope in the DC Universe. He's one of the sweetest people on the planet, constantly compassionate and polite, and much less Hot-Blooded than the rest of the Arrow Clan.
Mr. Tawky Tawny in the Shazam! franchise sometimes causes a scare for being a humanoid tiger, but anyone who takes the time to know about him will realize that he is a perfect gentleman and a wonderful person to know.
Captain Marvel is this in most incarnations, despite a fairly rough childhood. Billy Batson is usually a really sweet guy, often remarked on by other characters as being polite, friendly, kind and 'sunny'. And despite being able to turn into a superpowered demi-god, he is just as polite, friendly and kind as Captain Marvel. The Nu-52 version of Billy averts this, being a Jerk with a Heart of Gold, with said heart being buried waaaaay down beneath a whole lot of jerk.
Starfire is sometimes this. She comes from a culture of love and often helps her fellow heroes with their personal problems. Although she is often also a case of Good Is Not Soft.
Most incarnations of Superman's pal, Jimmy Olsen are kind and friendly. In one Silver Age comic, he even offers to kill a story already researched and put together simply because it bothers his grouchy bossnote the story (falsely, it turned out) suggested Perry's son Hank had gone delinquent. He is also arguably Superman's most loyal supporter, cemented by various storylines, including Truth.
Superman himself. The essence of his character is being a shining example for others to look up to.
Take notice of something, dear reader: the lack of examples in his entry is actually telling of how much of a Nice Guy Superman is. As his whole career is a succession of examples of the trope being played straight, the whole page could be filled with such examples until it becomes so bloated that it breaks down. Which is something Superman wouldn't like. Thus the deliberate lack of detailed examples.
In spite of her angst and anger issues, both of which she outgrows, Supergirl is, in her own way, every bit as sweet and kind as her cousin.
Even during the events of Red Daughter of Krypton, in which she was briefly an angry berserker, she was the nicest, most considerate and most tactful of all the Red Lanterns.
Hollis Mason, in Watchmen, doubles as the Only Sane Man. He's humble, friendly, and took up his career because he wanted to be a superhero. About the only thing he did in the story that could even count as mean was calling out a few people in his autobiography, and even then, the only person who really got it was The Comedian.
Wonder Woman was designed to be an embodiment of love and peace in The Golden Age of Comic Books who first tried finding peaceful solutions and reforming her enemies compared to the more aggressive methods of her male counterparts. Most incarnations since have followed suit by making her surprisingly humble and an All-Loving Hero that can find compassion even for the most vile of her enemies.
The Legend of Wonder Woman (2016): Wondy retains her usual All-Loving Hero status, and Steve Trevor is a genuinely kind, humble, honorable, self-effacing guy, which meant that when a ritual was done to magically pull in an innocent to sacrifice he was the one the magic grabbed.
