This is tough for me, because to be honest I haven't used that many published adventures over the years. I've used them more in the last couple years, during my return to roleplaying, but this still only amounts to a handful of adventures.
And frankly, most of the adventures I used were serviceable rather than extraordinary. Meaning, I embellished them a lot, which is my philosophy on how to run published adventures. I use them as a foundation, as inspiration, and rarely rely on boxed text, the exact monster stats, etc. I suppose that's how most GMs worth their salt run published adventures, so there's nothing exceptional about that.
If I have to choose, I'd say that running The Keep on the Borderlands on Free RPG Day 2011 was the most fun I've had running a module.
I think most of my enjoyment came from knowing that I was running a classic adventure, if not THE archetypal adventure. Yes, I know I was basically using AD&D (via Castles & Crusades) to run a module for Basic D&D, but whatever! We all know it just takes a little bit of tweaking to do such a thing.
Another joy came from the simple act of reading The Keep on the Borderlands. Even the name of the adventure is evocative, conjuring images of a remote outpost on the edge of an unexplored and dangerous wilderness. As stated, I didn't use many modules as a teen, so this was an opportunity to go back and read something that I missed back in my youth. This is especially true because I started out with AD&D, and much later than many bloggers (I got into RPGs in the late 80's).
So, Keep on the Borderlands is definitely my favorite adventure!