Probably Age of Worms. Mostly because it's the only long campaign that I actually played all the way to the end. Favorite campaign I ever ran: Demons in the Mist, which I largely made up on the fly as it was happening. It was pretty gonzo and had stuff like "The Jungle of Naked Fast Times era Phoebe Cates lookalikes who ride dinosaurs into battle against semi-civilized gorillas" and which, for a little while, gave players extra action points if they could quote 80s pop songs and make it sound unforced and like it actually fit. I had all kinds of weird ideas there; on a whim, I said no standard races (except human) and no spellcasting classes. I ended up with a human swashbuckler, a half-orc barbarian, a shifter soulknife, a hobgoblin rogue/fighter, another shifter of... I can't remember which class. It was a fun, crazy, swashbuckling game that felt very little in some ways like D&D as people know it.
The campaign that I regret not being able to finish the most is Horror on the Orient Express from the Kickstarter remaster from about ten years or so ago, or whenever that fulfilled.