Hmm, that's a damn good question.
Well, here are a few in no specific order:
1) Three straight Memorial Cups in EHM 2004, as well as four in five years.
2) Beating Metal Slug 1 co-op with my buddy, making it the first game I beat on my cabinet (and effectively on the Neo-Geo)
3) Kind of a weak one, but beating the Splinter Cells without killing any enemies (unless forced to)
4) Beating Panzer Dragoon Orta without dying one night
5) Getting my wife, someone who had only ever played an NES and her Intellivision as a youth, interested enough in games while dating to buy a Dreamcast, GBA and a few others. Now married, we both play on my systems but she researches and and buys her own games and wants to play stuff on my cab, etc.
Looking at them, I'd say the fifth one is my greatest achievement, the second one is the most nostalgic and the first one was the most difficult one.
I'm sure I'll remember more later.