Age of Worms finale (3E, 20th), "Dawn of a New Age." Doesn't get much more epic than looking back at your 1st level selves entering a cairn in search of adventure and culminating with facing down one of the game's most iconic demi-god bad guys (after having taken down the original dracolich) to save the world.
Savage Tides finale (3E, 20th), "Prince of Demons." Same thing except you get to face down the Prince of Demons (and not the watered-down version in 5E) at the head of an alliance of demons that you all recruited (including some of the big names). Great finale with some fantastic ways it could go.
A Paladin in Hell (2E, 15-20). Heroic premise. Paladin's funeral and his temple is dragged to the Nine Hells, so the PCs must right this horrific wrong by commandeering a demonic ship to sail the River Styx right into the mouth of endless enemies. Great art, great challenge, good odds not everyone is going to make it back. I can't give this adventure enough good words,
so someone else did.
It's really rare to get a campaign to this level, unless you skip ahead and begin at epic levels, and much of what makes most finale modules great is the culmination of your efforts. Obviously the adventure paths would be less special if you jumped in simply to battle Kyuss without having invested 1.5 years of your life into the story. "A Paladin in Hell" is pretty special for being stand-alone epic and all-around cool.