The scene you get if you manage to defeat both armies in "Make War, Not... Oh, Wait". It's very hard to get (doing so requires killing exactly 999 Frat Boys and 999 Hippies EACH, then exiting the map, adventuring there again, and using a flaregun in the last battle), but doing so provides you with a truly EPIC scene, as well as the Order of the Silver Wossname, a very nice piece of equipment.
Hobopolis is full of CMOAs, as it is a dungeon designed to be taken on not by a lone player but by a clan (however, each boss must be fought by a lone member of the clan). With every 250 regular hobo kills up until 1250, a new sub-area unlocks, each with its own boss, which should optimally be defeated as each boss left alive makes the final boss that much tougher. (Good luck—each of those sub-bosses is at least as tough as the Final Boss of the game.) An additional 250 unlocks an event that allows hobos to be killed quicker, and then it's another 1500 on top of that (that's 3000 total) before Hodgman, the Hoboverlord, is unlocked. By far the most epic quest ever.
The "Future" questnote accessible by using an empty agua de vida bottle by mere nature of the fact that the events are the result of quests undertaken in two other time periods, has a certain degree of awesomeness to it, albeit in a rather cheesy way.
Every Crimbo (Christmas) is Awesome, ending in a Crown.
2007: Fighting the 12 days of Crimborg, collecting a full suit of biomech armour and a Voltron toy so fully articulated that it effectively stabs, stomps, shoots, and strangles, before 'pulling the lever'' on Father Crimbo's reanimated cybernetic body.
2008: The Crimbomination battle. That is all.
2009: Elvish spies, penguin gangsters, and a Final Round with The Crimbomination, in which you sing him to death. Or friendship and the power of Crimbo.
The description of "Trusty", the Avatar of Boris axe, may be the most metal thing Jick has ever written:
Not every magical weapon is forged of meteorite iron under an unusual planetary conjunction, inscribed with gilded runes of ancient power, and imbued with supernatural strength and sharpness through mystical rites and sorcerous incantations. In truth, many of the most powerful weapons of lore are possessed of far humbler beginnings — common metal, torn from an enemy's grasp in a dire emergency. If the warrior survives the day, the weapon will likely be kept. Polished, sharpened, and re-sharpened, it will be carried from battle to battle, becoming as much a part of the man as his own arm, and as his name rises from warrior to hero to legend, so too will an aura of reverence and awe begin to surround the blade. Legend and belief are powerful forces, and it should be no surprise that a powerful artifact might have become powerful simply by dint of everyone believing it to be powerful. That is, after all, where the gods came from.
The ultimate enemy for 100% boredom in Crimbo 2010? The Best Game Ever. Because you can never be too meta.
The final battle with your nemesis.
Everyone's pretty much in agreement that The Bone Star was a CMOA both for the event it was a part of and especially for Jick's art. They even made shirts with the picture.
Crimbo 2012 capped off with a Humongous Mecha battle inspired by the likes of Voltron and Power Rangers. Everyone loved it so much that one of their favorite rewards from the event was a book that let you replay it.
As of February 2015, Ed had become so popular that a new challenge path was added: Actually Ed the Undying, where you get to play as Ed himself, in a post-Sorceress world where he's trying to track down the adventurer (i.e., you) who stole the Holy MacGuffin from him. (Hence, the "Actually" in the path name, as opposed to the "Avatar" paths for other legendary figures)
Of note is the conversation between Ed and the Adventurer before Ed lays the smackdown on them.
Adventurer: Are you going to do a super-cliché villain speech, like 'We're not so different, you and I'?
After defeating Yog-Urt, one of the possible bosses of the Mer-Kin Temple. The Adventurer becomes the focal point for the hatred of the Elder Goddess of, yes, Hatred, manifesting it in an all-encompassing rage and abomination at everything around him or her. Then, after that - and brutally murdering said Elder Goddess, of course - he or she GETS BETTER. The Adventurer shrugs off possession by an Elder Goddess's hatred with little more to-do than she or he shrugs off any other negative status ailment, because Ed and Krakrox aside, she or he is the biggest badass who ever lived.