First dragon your character slew (or some other powerful monster.)
Honestly, I've spent so little time as a player over the years that I can't recall any character of mine ever slaying such a mighty foe, but I do remember the first dragon that was slain by my players.
I was a bit disappointed that there were no dragons in the Keep on the Borderlands module. The second "D" is for Dragon, after all. So when I started expanding the wilderness map, I placed a swamp south of the road which was the lair of a black dragon, and a bluff some way farther along and to the north from which a red menaced the land. The garrison of the Keep warned the party about the red, and knowing that red dragons are fearsome monsters indeed, they were loath to venture that way.
The black, though, was a reclusive beast, and his presence in the swamp was not common knowledge. The swamp, appearing from the road a far greater mire than the little fens east of the keep, drew the party's curiosity, and they headed in. By now they were mostly 3rd level, with one 4th level character in the party, and felt pretty confident about dealing with lizard men and such.
The dragon's "lair" was in the open, on the shore of a stagnant mere, where it slept curled around a great stone chest that held its treasure. The party approached from the far side, and saw what seemed to be the dark bulk of a strange mound. They approached cautiously, circling the shore.
I don't remember how things went down from there, other than that they found themselves pinned between an angry dragon and the sodden muck of the pond, and that the dragon got at least one breath weapon attack in, because I recall having to look up the dimensions of a black dragon's breath line. The party took some pretty significant damage, but somehow everyone but the dragon survived the battle. My players were all pretty excited, and they certainly weren't disappointed with their first Type H treasure hoard, either.