Wow! This IS a hard one.
I don't remember ever slaying a dragon. Is that sad?
I think, years ago, I was playing D&D via the US Mail. My friend JD and I would write up detailed "stories" for each other, loosely based upon what each other's character would do.
He played Sharky Dangerthorn, a ranger and I played Robyn Hoodwynk, a monk.
I think there was an evil white dragon (Eversleet?) that Robyn fought and defeated. I'm just not sure.
Most of my D&D play has been as DM all these years, so, I don't have a lot of "big kills" in my memory banks.