Mine would be defeating Kefka for the first time in FFVI. After a year of flying around looking for everything I decided it was time and took him down.

Finding all all endings for Chrono Trigger. This was my replacement for FFVI and like VI I never put it down until I thought I found everything. It was in the days before walkthroughs, and found out about multiple ending in Nintendo Power and was convinced to see them all.