You sure? Really? Okay...
Mine isn't so much a single dream as it is a series of dreams linked together by a series of plotlines, a vast cast of characters, and recurring settings that started when I was about seven and kept going through lucid dreaming. You know how you sometimes have a really cool dream and wake up before you see how it ends and it's really frustrating? Well, it's sort of like that, only every time I would have a lucid dream, I would try to finish what the last dream started or revisit a cool place from another dream. Over time, things just wove together, building a really, really weird world, with everyone knowing that they were dreams inside my head and acting accordingly.
Season one occurred over the course of elementary school and revolved around being the leader of a gang of Lost Boys-esque magical children that lived in an invisible amusement park in the middle of the suburb my family lived in at the time. Adventures were pretty much doing whatever I wanted with little continuity, though a recurring side-story of a bunch of aliens disguising themselves as humans and kidnapping people off the street and taking them to their underwater laboratory started to develop.
Season two primarily took place in middle school and early high school, and revolved around the same character, only now it was mostly about him hunting down all the nightmares that had scared me as a kid and blowing them up, first solo and then as part of a SHIELD-esque government agency. Primary antagonists included the Nightmare King and the leader of the aforementioned aliens. Also marked the part where I started switching viewpoints between hero and villain, leading to some later dreams where I would act out the part of the nightmares.
Season three happened from late high-school through early college, and the focus shifted away from the previous hero to the nightmares themselves. The primary plotline involved a rouge cannibalistic nightmare seeking out some source of power located within my high school building for his own purposes, sometimes assisted by an evil twin of the previous protagonist created by the now defeated aliens. Other prominent characters involved an informant for the aliens who had switched sides and now worked for a place called the Safe House as a sort of witness protection thing and an undead vigilante who was a former nightmare himself before regaining his free will and joining the good guys by way of penance, and once had a whole series all to himself about him tracking down and killing his old partner to avenge his murdered ex-girlfriend. Who so happened to be River Tam from Firefly. Um, anyway, it ended when the rouge nightmare had a final showdown with the protagonist from the previous two seasons and defeated him, leading him to use the power he sought to, well, get really meta and tear his way out of my subconscious and take an elevator into my conscious mind, with the dream ending when the doors opened.
Also, there was something about a giant worm eating people in backyards that took up a few weeks. Nothing important, but it was memorable.
Since then, I've tried to get a new storyline going here and there, but nothing's really clicked. I unfortunately don't dream as often as I used to (blame my sleeping meds), so it's hard to get any steam. I guess in the end, the bad guy won after all.
So there you go. Good luck!