One of my earliest most memorable moments was defeating Metroid Zero Mission for the 1st time and watching the credits roll. It was the 1st time I really beat a video game because most of what I'd played previously had been arcade games which didn't have endings.
Entering Ganon's Castle in LOZ:OOT for the 1st time. The atmosphere from the game's final dungeon felt clear.
After 20 failures, I finally beat Normal Mode on Touhou: Mountain of Faith.
Defeating Mem Aleph the final boss in Shin Megami Tensei: Strange Journey. The whole game was completely brutal, but at lvl 68, my party of demons wasn't strong enough to deal enough significant damage to the final boss before taking on her extremely strong physical, elemental attacks, as well as nearly 100% guaranteed 1 hit KO spells (and in this game if your MC dies, you lose)
As well as navigating through the Great Palace of Zelda II, one of the largest video game dungeons I know of, where taking a wrong turn will put you on a long path that just leads to a dead-end and you have no way of knowing as you get killed by bird knights and the overall other bird themed enemies of this palace. Then once I found the right path, defeating Thunderbird which was not easy and beating the true final boss after that thanks to exploiting a bug with the final boss. (Call this unfair, but for a game as extremely difficult as Zelda II, it felt justified in order to finally beat the game)