Crash Bandicoot 3, 3rd level, 3 years old. Not only is that a triumvirate of trinities, it's the first time i beat any level in any video game ever.
(Not me playing)
My dad and I have played every Jak and Daxter game together, with the exception of Jak X. The day we beat the final boss of Jak II was Friday the 13th. I've never been superstitious about that day, but it was interesting to note.
From Jak 3, the scene where
Kingdom Hearts, killing Maleficent in her dragon form. Considering i played when i was six, that felt like a huge accomplishment.
Pokémon Crystal, looking at my save file to see that i had clocked in 100 hours. I had never had tangible proof that i played a game for that long before. Also, that's the only game i managed to complete the Pokédex in, not counting Celebi or any gen 1 exclusives.
Sonic Adventure 2 was the first time i cried at a video game. Admittedly, the game's hard to take seriously at my current age, but it still was horribly depressing at the time.
Buying Soul Calibur IV for the sole purpose of using the character creator. I quickly tracked down Soul Edge through Soul Calibur III, and Legends, because i found the character dynamics very interesting. Then, Soul Calibur V killed the franchise to the point that i doubt it can recover without a reboot.... Anyway, that's how i discovered that i enjoy fighting games.
Shin Megami Tensei II, killing the Christian God. When Christianity was the source of your depression, this is very cathartic.
Dark Souls, killing the Taurus Demon for the first time. I kept two-handing an axe and trying to hack it to death. I got mad and stopped playing for a while. When i came back, i found the ladder, and drop attacked the mofo, and then finished him off with an R2 to the crotch.
Also from Dark Souls, i went into Sen's Fortress while human, still being in that form since summoning Mildred to help against Quelaag. Then i got invaded. He attacked me while i was on a bridge with pendulums swinging above. I blocked his attack, then kicked him into a pendulum, causing him to fall off and die. That was the first time i killed an invader.
Getting Great Wolf Sif to low enough health that the poor guy started whimpering and limping towards me, falling over as he attacked. I honestly felt bad for killing him dead, even though he'd done the same to me many times already.