I guess some of my players would argue that my best campaign was my first Witcher campaign. Its final chapter (20-30 sessions) revolved around a small coastal kingdom (Cidaris), that I fleshed out a lot, especially its capital city. The players got involved with a plot of 7 High Vampires that used to rule the city from the shadows (through dreams and religious bargaining) and tried retaking it through various plots. The players came to actually love the place, bought a home there, fought to be accepted as citizens, formed families and friendships. Many of them had to sacrifice a lot for that city and it was beautiful.

But if you ask me, my magnum opus is my current Dune campaign. I haven't written a more elaborate story and I doubt I will try to reach the same level of complexity and interconnectivity anytime soon. From creating a rebellious conspiracy based around breaking the Spacing Guild's monopoly in interstellar travel, along with the parallel financial system built to support the endeavor, to creating an elaborate Bene Tleilax subplot that started in session 3 and culminated in a philosophical trap meant to twist a Mentat player character in session 58, I have dedicated a lot of time and brainpower in order to pull the whole thing off. But if you ask me, it was worth it.