For me was without any doubt finishing Out Run on the arcade coin machines, then Arkanoid and later R-Type.
I remember skipping school sometimes to be able to play on the arcade saloon on the not-so-much busy hours, so I could have some peace without everyone putting their breath behind me ears..
... and in the process not lose money!
In the Amiga was finishing Rocket Ranger for the first time and later on it Came From the Desert. Many hours invested in these two awesome games, specially Rocket Ranger where i remember fondly the joy that me and my brother had that day.
Haaa memories. Today when I finish another PC game is just: was nice, another one finished. What's next?
I think having finished Raiden on the league could come as the more recent sense of accomplishment that I have had from many years now.