I was like 7 and didnt know anything about legendaries and stuff.
I was below blackthorn city, just training with the gravelers and skarmories (it was silver), and i was in my bed, almost paying no atention to the screen (i was seeing it, but not actually "looking" it).
Anyway, when a battle started, just as a black sprite was sliding on the screen, i noticed it was weird: yeah, in those nanoseconds i tough "oh, so i can find ponytas here? Cool!", but when the sprite cleared and the pokemon cry sounded, i couldn´t but look paralized at the blue beast-like monster. My heart stoped. I pressed the A button..
"A wild SUICUNE appeared"
That thing was level 40, what could my haunter do? Not knowing what to do, i threw an ultra ball. He freed.
"The wild Suicune used ROAR!"
And after a few second recovering for the impresion, i opened the pokedex and rushed from the very first poke until i saw him almost at the end, and had one of the biggest reliefs of my life when i pressed "Area" and he was near goldenrod.
That week i stared my haunt for the 3 dogs (soon Entei appeared in the grass in middle of the lake next to Mahogany, but it wasn´t until weeks later that Raikou appeared under Goldenrod)
Btw, suicune became like my favorite poke back then (he destroyed everything thyplosion couldn´t), and i loved his back sprite.
Another good ones were:
-Evolving togepi at level 19
-Learning how to fuse pokes (for me, it was even better than cloning, in wich i was a true master btw). I spent days fusing almost everything with the red gyarados and beetwen them to make my own shiny pokes (witn impossible movesets, of course).
-Learning about the egg moves, and breeding in general (whats not to love in a level 5 charmander with flamethrower?)
-Beating the E4 over 300 times (not kidding), and well, battling with my best friend and generally exploiting the glitches.
Oh, and last week when i turned my game boy on and realized gold´s battery is still alive
Yes, G/S/C are the best.