Watchmen was written as a deliberate Deconstruction of more idealistic comic book superheroes, the idealism of superheroes, and the superhero genre in general. It shows what would really inspire people to go out in ridiculous, often-times skimpy uniforms and beat the crud out of other people, and one of the characters quite intentionally crosses the line separating idealistic superheroism from deluded vigilante action.
Alan Moore later felt that, partly as a result of the popularity of Watchmen, later superhero comics completely missed the point and focused too much on the wrong things, going too far to the other side of the scale and forgetting to retain some level of idealism and fun in the process. In an effort to remedy this, he created Tom Strong, a more idealistic superhero series, in order to even the scales a bit. He also did a landmark run on Supreme and wrote 1963 in a further attempt to reverse the trend.
The Punisher is a cynical character in a shared universe; his "rightness" fluctuates wildly depending on where the series he appears in falls on the scale. In his MAX series, a more adult comic, there is little question to the effectiveness of his actions, and his antagonists are usually consistently pure evil (The Slavers), but in the mainstream comics, he is often shown in a less favorable light.
Throughout the events of Archie Meets the Punisher, Frank monologues on Riverdale's inability to deal with the scum he handles on a daily basis, while at the same time wishing he could have grown up with the quiet, friendly lifestyle that they enjoy.
JLA Classified # 3. Superman tells the International Ultramarine Corps (a pastiche of cynical superhero teams) that "These 'no-nonsense' solutions of yours just don't hold water in a complex world of jet-powered apes and Time Travel," and gives them the chance to go to a baby universe troubled by "cynical" problems.
Heck, Grant Morrison in general seems to lean towards the idealistic side of the scale. Final Crisis especially slams hard against the idealism side by the very end what with the representation of the dark, cynical kick comics had been on being defeated by (essentially) the manifestation of the upbeat, optimistic, and fantastical comics of the Silver Age.
Often, who's writing for a character in a comic book determines where on the scale that character falls. In some books, Batman is one step up from the Joker. In others, he's almost as much of a boy scout as Superman. Since the writing duties of a comic series can change from issue to issue, this can be slightly disorienting, as the reader doesn't know from one Story Arc to the next if the book's star is going to be a jerk or a hero. In this scenario, it's also a form of Writer on Board. This also applies to any long-running TV series with frequent writer changes and a dramatic bent.
The Superman/Batman series manages to successfully show both titular characters on their comparative scales and makes a point of showing neither as more correct than the others. At one point, Batman states that Superman's selfless idealism is the reason why he should be considered a hero. If Superman ever let himself sink to Batman's cynicism, it wouldn't be pretty. However, it has also been stressed that, of the two of them, Batman is the more alien of the pair, mostly because of his cynicism.
Oddly enough, whenever he's by himself (in the incredibly Crapsack World of Gotham), Batman tends to be less of a cynic, but becomes much more of one when he's around other characters and has to fill that niche.
The scale is examined very effectively in the Superman comic "What's So Funny About Truth, Justice And The American Way?" Of course, being about the original Cape himself, the conclusions it raises fall squarely on the idealistic side of the scale, but it's a well-written story nonetheless.
For a good long while, a major selling point of the Marvel Universe in general was that their characters were more realistic (read: cynical) than in The DCU; of course, they were often just as implausible in nature, but Marvel's characters often possessed more character flaws and personal issues than the idealistically "perfect" heroes in DC. These days, given forty odd years of Character Development and competition since Marvel first hit it big, this distinction isn't quite as significant as once it was; unfortunately, both companies have a tendency to instead plunge into whichever side of the scale that will make their characters more angsty.
The formerly-canon version of Superman has killed precisely once, during the Dark Age, in order to Shoot the Dog on three Kryptonians from an Alternate Universe. Since then, writers have either ignored this, or have him regard it as a mistake that made his self-imposed prohibition against killing even stronger in response. As of current canon, Superman has never killed anyone.
Wonder Woman on the other hand in modern times is a classically trained warrior who is ready to use deadly force if necessary. For instance, former ally Max Lord gains mind control powers and uses them to make Superman try to kill everyone; when Wondy asks him what will make him stop, Max tells her to kill him, and she does. The event is broadcast worldwide to the public by Max's spy cameras and severely hurts Wondy's reputation.
There are a few authors who will completely ignore this principle when writing in the DCU; Frank Miller is probably the best-known example.
There is one current superheroine with which this completely does not apply: Manhunter. In her first appearance, she killed Copperhead and has never regretted it. In fact, even people who know her secret identity aren't bothered by it - probably because of the fact that Copperhead was a mass murderer and had just slaughtered a bunch of cops. She's even teamed up with Oracle, been the lawyer of Wonder Woman, and has consulted Batman and Superman for help before.
Similarly, the Marvel Universe seems to take All of the Other Reindeer as a guiding principle for their sustained "realism", and has since The Seventies. DC is leaning toward this of late as well. I understand there is prejudice in the world, but one may wonder how much distrust of the abnormal can lead people to abandon all ethics, principles, and even senses of self-preservation.
This "realistic" approach was even reflected in the settings of their stories; whereas DC's comics were (and mostly still are) set in fantastic (and fictional) locales such as Metropolis and Gotham City, Marvel set its comics in the very-real streets of New York City.
If anything, since the 80s DC has become more cynical than Marvel. And Marvel's New York is no more real than Gotham or Metropolis just because it shares its name with a real world city.
Dan Slott's Pre-Civil War work in Marvel falls on the idealistic side. He even has Nighthawk say that he keeps being a superhero because it's fun.
Judge Dredd falls squarely into the cynical side of the scale. Several storylines examine the scale, with the cynicism of Dredd and the Judges contrasted with the idealism of pro-democracy activists seeking an end to the authority of the Judges and the return to democratic government and the separation of powers to the world of Mega-City-One. After a democratic referendum, democracy ultimately fails, validating Dredd and the Judges' viewpoint. Even the most committed activists either resign themselves to defeat and give up in complete disillusionment, or become fanatical and ruthless terrorists, just as bad, if not worse than the Judges they despise.
Scott McCloud's Zot! is a study in contrast between Zot's Earth of "far-flung future of 1965," an idealistic world with Crystal Spires and Togas, where everything's pretty much perfect except for some supervillainy that Zot always stops, and Jenny's Earth, our Earth, which falls into the normal realm of cynicism. In the first story arc, where Zot visits Jenny and he decides to go to a bad part of town and stop a purse-snatcher, not only does he get badly beaten, but there is a crowd of onlookers who do absolutely nothing. Even though this doesn't discourage Zot at first, after he fails to rescue some from a fire (it having been previously explained that Zot "never loses" because he believes he can never lose), he starts thinking that Jenny's Earth really isn't that good and leaves. Zot does eventually return, however, and his essential optimism and faith in human decency never seriously weakens, and even on Jenny's Earth is paid off, from time to time; similarly, Jenny's cynicism about the world, whilst justifiable and not invalid, can be misguided.
In the Idealism extreme, we have Piffany from Nodwick, who believes that everything is goodness and light, despite the evidence displayed by her fellow party members. Nodwick himself is justifiably much more cynical.
As a whole, Kurt Busiek's Astro City tends towards the Idealistic side of the scale, with heroes who tend to be noble and selfless models that the citizens admire. But just before you peg the series as hopelessly idealistic, some cynicism sinks in, such as the "shame" felt towards the Silver Agent (who was framed by the government and executed to show that they could control superheroes), the betrayal of El Hombre, and the entire Dark Ages story arc. Ultimately, though, idealism wins, and even former super-criminals can redeem themselves if they try.
Sin City is heavily cynical but so over-the-top that it's part of its charm.
Boondocks is a relentlessly cynical satire comic about Black people and the unstoppable nature of corporate greed and Blaxploitation.
An excellent illustration of the divide between DC and Marvel comes in JLA/Avengers, where the two teams end up in the others' universe. Captain America sees the way DC's civilians celebrate their heroes and assumes they've set themselves up as tin-pot dictators; meanwhile, Superman sees how bad off the Marvel universe is and decries their heroes for being selfish and not helping the common man enough. The pair actually comes to blows over this (to the confusion of their respective teammates), and it's later revealed that the stress of their two universes merging is having a negative effect on the two men since they're so strongly tied to their respective worlds. The two manage to have an honest talk about the concerns of going too far or not doing enough, and when they part ways they agree that above all else, the important thing is that they try their best.
Kick-Ass is about as cynical as it gets, even more so than Watchmen. Dave is a loser, Big Daddy is a complete fraud, Hit Girl is lied to by her father about her mother dying, and not allowed to have a normal childhood, and everyone else except for maybe Dave's father is a scumbag of one sort or the other (Katie is a shallow bitch, Red Mist is completely unsympathetic unlike in the film, his father is a evil, etc). Despite all this, it's incredibly funny. Many people preferred the movie adaptation since it toned down the utter bleakness of the comic book, but taken on its own terms, the comic is a great Black Comedy.
That said, Mark Millar is very cynical comic book author. His other most famous work was Wanted which also falls in the far end of the Cynical side as well as The Authority.
Probably the one book he did that falls squarely on the idealistic side is Superior, a book about a 12-year-old boy suffering from multiple sclerosis who gets super powers, loses them, and in the process, learns to come to terms with his disability.
One of the draws of the Green Lantern and Green Arrow series was this, Lantern as idealistic, Arrow as cynical. This is brought up later in Green Lantern: Rebirth, when GA tries to use GL's power ring to defend himself, only for Sinestro to smack him down and mocking his will as being too cynical to even get the ring to work. So, idealism isn't so bad...
The Walking Dead is an extremely cynical zombie survival story. When they talk about the walking dead, it isn't about the zombies but humanity who is simply circling the drain.
The name Garth Ennis is synonymous with the word "ExtremeCynicism". Partially due to Garth Ennis' complete aversion towards superheroes, he enjoys creating works that are dark and brutal.
More Than Meets The Eye and Robots In Disguise, two Transformers comic series, are sister series set in the same universe but fall on completely opposite sides of the scale. More Than Meets The Eye is very idealistic with funny characters, an emphasis on action and humor, heartwarming moments, and heroes (and sometimes even villains) who do heroic things. Robots In Disguise, on the other hand, is quite cynical with Fantastic Racism, heroes who sometimes do or consider doing terrible things in the name of the greater good, and truly heroic characters like being shoved aside or having no real impact. Both series also have idealistic or cynical characters added to the main cast (MTMTE has Tragic Hero and Jerkass Whirl and the constantly miserable Crankcase alongside idealistic characters; RID has Metalhawk who sees the best in everyone, lovable Mad Scientist Wheeljack, and Big Good Optimus Prime alongside mostly cynical characters).