Ok, this was a very hard question. There are a lot of fics that still make me laugh, fics where I'm proud of the atmosphere I was able to achieve, and still other fics where the story's flaws might be too apparent but the world it creates is one I hope to revisit someday.
In the end, I have to go with
What's Bugging You, a fic from waaaaay back which can be summarized as "A big bug gets into Lois and Clark's house". The sheer, unbridled absurdity of the premise speaks to a level of uninhibited writing that I struggle to recapture these days. In one sense, it was my pinnacle. Why is the roach the size of a person? How and why did it literally knock on their door? Even I don't know, and I'm the author! It's just there, unexplained, and the characters have to roll with it, and nevermind petty things like "logic". Also, I can't help but love the mental image of a sofa-cushion fort.
