First Dragon Your Character Slew (or Some Other Powerful Monster).
I'm a bit embarrassed by this but I can't remember the first dragon I slew. I remember facing them and other powerful creatures in 2nd edition, but I can't remember specific encounters. It kind of makes me sad, but most of my memories of anything before 3rd edition.
Most of my gaming took place during the 3.0/3.5 days. The first truly memorable fight against a powerful opponent I can remember would be when one of the parties Ulic was traveling with took on a balor. This fight was particularly frightening because Ulic was several levels lower than the rest of the party. I believe he was around 12 or 13 and the rest of the party was 16+. The battle was a long and there one of our rogues, Alexander died. Ulic did the best he could, providing aerial support. When the balor finally died his explosion almost killed Ulic as well. I took more than half hitpoint damage, while flying. I fell unconscious and many feet. Luckily I woke up in time and slipped on a ring of feather falling in time.