Karnov

I'll always remember because my friend and I had sooo much fun and kept getting farther and farther until an epic climb of increasingly cooler stages and finally defeated the main boss. We didn't know what to expect! We never finished a game before! What's going  to happen? then suddenly we saw it!

Congratulations!

uh, that's a tough question, as a kid I recall playing LOTS of games on my PC, but I can't rememember if I ever beat any of them, on the other hand I know that the first game I 100% completed was Pokémon Platinum

First beaten? I have no clue. First 100%? Super Mario 3D Land. I was 10.

Hmm, I can't recall, I think it was Super Mario Land on the Game Boy?

First game I beat was probably Pokemon Crystal for Game Boy Color.
As for first 100%, it was actually 101% on Jak and Daxter: The Precursor Legacy, I knew where all the power cells, scout flies and precursor orbs were in that game. I remember I could do 101% in about 6 hours from start to finish. Needless to say I loved that game.

At home, hard to say. Most of the early games I remember didn't have an end, like Tetris and rogue.
Perhaps Lemmings, definitely beat that, should still have the entire list of level passwords somewhere.

The first game I could remember beating is Mega Man 2, although this was with the aid of a game genie. The first game I can recall beating legitimately was Kirby's Dream Land.

If I recall correctly, it was Smash Melee

..unless you include games like Pinball, Solitaire or Minesweeper on PC :p

Panzer Dragoon Zwei for Saturn (First Beat)

Kingdom Hearts (First 100%)