Pokémon Gold, I completed it more times than I could count when I was a kid.
Always going for different teams in each run, I remember getting a Seadra to level 100 thinking it would evolve, I knew it was odd when it didnt evolve till level 60 but I thought Kingdra was so badass it would evolve to it only on level 100.
I was so obsessed with the Ruins of Alph, I couldnt get over how it was so mysterious and yet useless area, I spent hours trying to find some secret after getting all the Unowns and changing the radio stations, I even started walking around with 5 Unowns in my party spelling "Unown" and in the right order just to be disappointed in the end.
Even if I got so frustrated it's still my favorite Gen :D