The day I received my NES and Genesis.
Completing Journey to Silus for NES
Completing Contra without the 30 life code before any of my friends
Completing Shinobi for the PS2 on the hardest difficulty
The first time I finished Streetfighter that had the big ass buttons and then SF2 with Ryu
The first time I witnessed and did the raging demon in SFA
Finishing each Metal Slug Game accept 3 (it’s hard as fuck!) on 1 credit
Completing Magic Sword with 1 credit
Getting a million points for the first time SF2
Mastering all the characters in MK2 so that I could use Shang Tsung
The first time I played MvsC2 after it had been out for about a year (I was stationed in Italy and discovered it when I got back)
Finding out Samus was a woman on the NES and completing Super Metroid with 100% and in enough time to see her half naked.
Having completed every Megaman game and all of it’s spinoffs to date.
There are more, but those are the ones I can think of off the top of my head.